feat: add markdown links checker workflow

[jaybxyz]

Description

closes: #448

Tasks

• Add linkchecker.yml
• Fix broken links (None)

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #450 (9d729d0) into develop (aa1af26) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #450   +/-   ##
========================================
  Coverage    85.10%   85.10%           
========================================
  Files           23       23           
  Lines         2618     2618           
========================================
  Hits          2228     2228           
  Misses         226      226           
  Partials       164      164           

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