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/docs/PULL_REQUEST_TEMPLATE.md

@@ -14,7 +14,7 @@ Before you submitting, please check that:
   - [ ] Github's project assignment
 - [ ] You mentioned a related issue if this PR related to it, e.g. `Fixes #228` or `Related #1337`.
 - [ ] You asked any particular reviewers to review. If you aren't sure, start with GH suggestions.
-- [ ] Your PR adheres to [the style guide](https://wiki.parity.io/Substrate-Style-Guide)
+- [ ] Your PR adheres to [the style guide](https://github.com/paritytech/substrate/blob/master/docs/STYLE_GUIDE.md)
   - In particular, mind the maximal line length of 100 (120 in exceptional circumstances).
   - There is no commented code checked in unless necessary.
   - Any panickers have a proof or removed.

substrate/docs/SECURITY.md

@@ -1,3 +1,4 @@
+<!-- markdown-link-check-disable -->
 # Security Policy
 
 Parity Technologies is committed to resolving security vulnerabilities in our software quickly and carefully. We take the necessary steps to minimize risk, provide timely information, and deliver vulnerability fixes and mitigations required to address security issues.