Add a workflow to check documentations (#1151)

* Add a workflow to check documentations

* Fix markdown issues

* Install Ruby 2.7

* Finally, handle and fix GitHub templates

* Minor improvement in the feature request template

* Verbose mode to be sure all files are checked

.github/workflows/documentations.yml

@@ -0,0 +1,21 @@
+name: Check documentations
+
+on:
+  pull_request:
+    paths:
+      - "*.md"
+      - "**/*.md"
+
+jobs:
+  doc:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 2.7
+      - name: Install the linter
+        run: sudo gem install mdl
+      - name: Check files
+        run: make doc-check