Add markdownlint

Add a Markdown linter.

.github/ISSUE_TEMPLATE.md

@@ -1,3 +1,4 @@
+<!-- markdownlint-disable MD041 -->
 ### Expected behaviour
 
 <!-- Explain what you expected to happen. -->

.github/ISSUE_TEMPLATE/bug_report.md

@@ -2,6 +2,7 @@
 name: Bug report
 about: Create a bug report to help us improve the library
 labels: bug
+title: ''
 ---
 
 ### Describe the bug

.github/ISSUE_TEMPLATE/feature_request.md

@@ -2,9 +2,10 @@
 name: Feature request
 about: Suggest an idea for a feature of this library
 labels: feature-request
+title: ''
 ---
 
-### Is your feature request related to a problem? Please describe.
+### Is your feature request related to a problem?
 
 <!--
 A clear and concise description of what the problem is. For example: _It would be useful if [...]_

.github/workflows/lint.yml

@@ -37,3 +37,10 @@ jobs:
       uses: docker://rhysd/actionlint@sha256:89d3f90f82781dee3c8724651129634b08cf2241bbd67fcd02a1c5198119fc5e # v1.7.2
       with:
         args: -color
+
+    - name: Lint markdown
+      uses: DavidAnson/markdownlint-cli2-action@db43aef879112c3119a410d69f66701e0d530809 # v17.0.0
+      with:
+        config: '.markdownlint.json'
+        globs: |
+          **/*.md

.markdownlint.json

@@ -0,0 +1,4 @@
+{
+  "MD013": false,
+  "MD040": false
+}