Update installation page (#15737)

The installation page is updated with:

* updated Python/Kubernetes support policies
* added download information to installation page

docs/apache-airflow/installation.rst

@@ -263,22 +263,46 @@ If you don't want to install any extra providers, initially you can use the comm
     pip install "apache-airflow==${AIRFLOW_VERSION}" --constraint "${CONSTRAINT_URL}"
 
 
-Python versions support
-'''''''''''''''''''''''
+Support for Python and Kubernetes versions
+''''''''''''''''''''''''''''''''''''''''''
 
-As of Airflow 2.0 we agreed to certain rules we follow for Python support. They are based on the official
-release schedule of Python, nicely summarized in the
-`Python Developer's Guide <https://devguide.python.org/#status-of-python-branches>`_
+As of Airflow 2.0 we agreed to certain rules we follow for Python and Kubernetes support.
+They are based on the official release schedule of Python and Kubernetes, nicely summarized in the
+`Python Developer's Guide <https://devguide.python.org/#status-of-python-branches>`_ and
+`Kubernetes version skew policy <https://kubernetes.io/docs/setup/release/version-skew-policy>`_.
 
-1. We end support for Python versions when they reach EOL (For Python 3.6 it means that we will stop supporting it
-   on 23.12.2021).
+1. We drop support for Python and Kubernetes versions when they reach EOL. We drop support for those
+   EOL versions in master right after EOL date, and it is effectively removed when we release the
+   first new MINOR (Or MAJOR if there is no new MINOR version) of Airflow
+   For example for Python 3.6 it means that we drop support in master right after 23.12.2021, and the first
+   MAJOR or MINOR version of Airflow released after will not have it.
 
-2. The "oldest" supported version of Python is the default one. "Default" is only meaningful in terms of
-   "smoke tests" in CI PRs which are run using this default version.
+2. The "oldest" supported version of Python/Kubernetes is the default one. "Default" is only meaningful
+   in terms of "smoke tests" in CI PRs which are run using this default version and default reference
+   image available in DockerHub. Currently ``apache/airflow:latest`` and ``apache/airflow:2.0.2`` images
+   are both Python 3.6 images, however the first MINOR/MAJOR release of Airflow release after 23.12.2021 will
+   become Python 3.7 images.
+
+3. We support a new version of Python/Kubernetes in master after they are officially released, as soon as we
+   make them work in our CI pipeline (which might not be immediate due to dependencies catching up with
+   new versions of Python mostly) we release a new images/support in Airflow based on the working CI setup.
+
+Installing Airflow From Released Sources and packages
+'''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+You can also install Airflow using the official sources and packages. Those sources and packages
+released are the "official" sources of installation that you can use if you want to verify the
+origin of the packages and want to verify checksums and signatures of the packages.
+
+The packages are available at the
+`Official Apache Software Foundations Downloads page <https://downloads.apache.org/airflow/>`_
+
+The |version| downloads are available at:
+
+* `Apache Airflow |version| sdist package <https://downloads.apache.org/airflow/|version|/apache-airflow-|version|-bin.tar.gz>`_ (`asc <https://downloads.apache.org/airflow/|version|/apache-airflow-|version|-bin.tar.gz.asc>`__, `sha512 <https://downloads.apache.org/airflow/|version|/apache-airflow-|version|-bin.tar.gz.sha512>`__)
+* `Apache Airflow |version| wheel package <https://downloads.apache.org/airflow/|version|/apache-airflow-|version|-py3-none-any.whl>`_ (`asc <https://downloads.apache.org/airflow/|version|/apache-airflow-|version|-py3-none-any.whl.asc>`__, `sha512 <https://downloads.apache.org/airflow/|version|/apache-airflow-|version|-py3-none-any.whl.sha512>`__)
+* `Apache Airflow |version| sources <https://downloads.apache.org/airflow/|version|/apache-airflow-|version|-source.tar.gz>`_ (`asc <https://downloads.apache.org/airflow/|version|/apache-airflow-|version|-source.tar.gz.asc>`__, `sha512 <https://downloads.apache.org/airflow/|version|/apache-airflow-|version|-source.tar.gz.sha512>`__)
 
-3. We support a new version of Python after it is officially released, as soon as we manage to make
-   it works in our CI pipeline (which might not be immediate) and release a new version of Airflow
-   (non-Patch version) based on this CI set-up.
 
 Set up a database
 '''''''''''''''''

docs/conf.py

@@ -255,9 +255,12 @@ def _get_rst_filepath_from_path(filepath: str):
     html_extra_path = [
         f"{ROOT_DIR}/docs/apache-airflow/start/airflow.sh",
     ]
-    html_extra_with_substituions = [
+    html_extra_with_substitutions = [
         f"{ROOT_DIR}/docs/apache-airflow/start/docker-compose.yaml",
     ]
+    manual_substitutions_in_generated_html = [
+        "installation.html",
+    ]
 
 # -- Theme configuration -------------------------------------------------------
 # Custom sidebar templates, maps document names to template names.

docs/exts/extra_files_with_substitutions.py

@@ -25,19 +25,28 @@ def copy_docker_compose(app, exception):
     if exception or not isinstance(app.builder, builders.StandaloneHTMLBuilder):
         return
 
-    # Replace `|version|` in the docker-compose.yaml that we produce in the built docs
-    for path in app.config.html_extra_with_substituions:
+    # Replace `|version|` in the docker-compose.yaml that requires manual substitutions
+    for path in app.config.html_extra_with_substitutions:
         with open(path) as file:
-            with open(os.path.join(app.outdir, os.path.basename(path)), "w") as output:
+            with open(os.path.join(app.outdir, os.path.basename(path)), "w") as output_file:
                 for line in file:
-                    output.write(line.replace('|version|', app.config.version))
+                    output_file.write(line.replace('|version|', app.config.version))
+
+    # Replace `|version|` in the installation.html that requires manual substitutions (in links)
+    for path in app.config.manual_substitutions_in_generated_html:
+        with open(os.path.join(app.outdir, os.path.basename(path))) as input_file:
+            content = input_file.readlines()
+        with open(os.path.join(app.outdir, os.path.basename(path)), "wt") as output_file:
+            for line in content:
+                output_file.write(line.replace('|version|', app.config.version))
 
 
 def setup(app):
     """Setup plugin"""
     app.connect("build-finished", copy_docker_compose)
 
-    app.add_config_value("html_extra_with_substituions", [], '[str]')
+    app.add_config_value("html_extra_with_substitutions", [], '[str]')
+    app.add_config_value("manual_substitutions_in_generated_html", [], '[str]')
 
     return {
         'parallel_write_safe': True,

docs/spelling_wordlist.txt

@@ -564,6 +564,7 @@ cgroupspy
 changelog
 charset
 checklicence
+checksums
 chmod
 chown
 ci