fix: markdown links, check markdowns on PR

[Zvikan]

What does this PR do?

• fix broken link in docs.
• adjust markdown link action to run on PRs against main only when .md files modified.

Motivation

• broken main check - https://github.com/aws-ia/terraform-aws-eks-blueprints/runs/7149823004?check_suite_focus=true
• it make sense to check this on PR before changes to main (unless I am missing something here).

More

• Yes, I have tested the PR using my local account setup (Provide any test evidence report under Additional Notes)
• Yes, I have added a new example under examples to support my PR
• Yes, I have created another PR for add-ons under add-ons repo (if applicable)
• Yes, I have updated the docs for this feature
• Yes, I ran pre-commit run -a with this PR

Note: Not all the PRs required examples and docs except a new pattern or add-on added.

For Moderators

• E2E Test successfully complete before merge?

Additional Notes

[bryantbiggs]

oh - we should chat about this. we had this here but it doesn't work when adding new addons or new pages to docs since its checking whats currently live and not what "would be live" so it fails PRs.

not sure what the correct route is - maybe we want that and its up to reviewers to know if thats a true failure or a false failure since its getting added in the PR

[Zvikan]

created #725 for testing purposes, looks like the PR check fails as expected but this is coming from a branch created from the same repo, need to test this against a PR coming from external contributor from fork