build: use mamba to install minian

* use mamba to setup environments during testing
* suggest using mamba to install minian in documentation.

.github/workflows/testandcov.yml

@@ -34,6 +34,7 @@ jobs:
         with:
           auto-update-conda: true
           python-version: ${{ matrix.python-version }}
+          mamba-version: "*"
           activate-environment: minian
           environment-file: environment.yml
       - name: Install test dependencies

docs/source/start_guide/install.rst

@@ -22,6 +22,19 @@ After you have created and activated an environment, you can install MiniAn with
 
 and Done!
 
+Alternatively, you can use `mamba <https://mamba.readthedocs.io/en/latest/>`_ to install minian, which usually provides faster speed when solving the dependencies.
+To do so, you first need to install `mamba`, either in minian environment or in your base environment.
+
+.. code-block:: console
+
+    conda install -y -c conda-forge mamba
+
+After this, you can use `mamba` as a drop-in replacement command for `conda` to install minian:
+
+.. code-block:: console
+
+    mamba install -y -c conda-forge minian
+
 Install from source
 -------------------