CI: markdown link checker (#7145)

* change (CI): markdown link checker

* Fix some invalid doc links (re-run of cargo-unleash gen-readme w/ fixes).

* Fix some invalid doc links

* Fix some invalid doc links

* Fix some links

* Fix some links

* Apply @bkchr suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Fix more links

* Fix more links

* typo

* Fix more links

* Fix more links

* Ignore valid link .. check wrongly sees it as invalid

* Fix style issue

* Fix style issue

* change (CI): update style guide link

* change (lib): suggestions

Co-authored-by: Dan Forbes <dan@danforbes.dev>
Co-authored-by: Steve Degosserie <steve@parity.io>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/.github/workflows/md-link-check.yml

@@ -0,0 +1,19 @@
+name: Check Links
+
+on:
+  pull_request:
+    branches:
+    - master
+  push:
+    branches:
+    - master
+
+jobs:
+  markdown-link-check:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: gaurav-nelson/github-action-markdown-link-check@v1
+      with:
+        use-quiet-mode: 'yes'
+        config-file: '.github/workflows/mlc_config.json'