🌱 Add broken link check: general and pr

[peppi-lotta]

These workflows can be called from the other Metal3 repos to test for broken links.
.github/workflows/broken-link-check.yml is for testing all md files in a repo
.github/workflows/pr-link-check.yml is for testing all .md files that have changed in a PR. This does not check all files but only the ones that have changed.

I've tested the pr workflow in situations listed below and it is giving the output I expect

• .md file is edited and doesn't have any link (pass)
• .md file is edited and has working links (pass)
• .md file is edited and has broken link (fail)
• pr does not have any edited .md files (pass)

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[tuminoid]

Just quick comment about the filenames at this point.