feat: add markdown links checker

[jaybxyz]

Description

This PR adds markdown link checker workflow to check all Markdown files for broken links every 24 hours.

closes: #41

Tasks

• Add linkchecker.yml
• Fix broken links

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[codecov]

Codecov Report

Merging #43 (4e31b03) into master (e21abdf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #43   +/-   ##
=======================================
  Coverage   79.85%   79.85%           
=======================================
  Files          12       12           
  Lines         283      283           
=======================================
  Hits          226      226           
  Misses         42       42           
  Partials       15       15           

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[jaybxyz]

Just realized that this PR is not up to date. So I just merged with the latest master branch.

[dongsam]

I don't think linkchecker is working at the moment in workflow, so I think we need to merge after checking that it is working by adding a broken link for testing.

[jaybxyz]

I reference Cosmos SDK's linkchecker.yml, and they seem to use scheduled runs to check links regularly in addition to checking links on every push or pull requests.

[dongsam]

LGTM!