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/primitives/allocator/README.md

@@ -1,6 +1,6 @@
 Collection of allocator implementations.
 
 This crate provides the following allocator implementations:
-- A freeing-bump allocator: [`FreeingBumpHeapAllocator`](freeing_bump::FreeingBumpHeapAllocator)
+- A freeing-bump allocator: [`FreeingBumpHeapAllocator`](https://docs.rs/sp-allocator/latest/sp_allocator/struct.FreeingBumpHeapAllocator.html)
 
 License: Apache-2.0