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/mlc_config.json

@@ -0,0 +1,7 @@
+{
+    "ignorePatterns": [
+        {
+            "pattern": "^https://crates.io"
+        }
+    ]
+}