feat(ci): add markdown link checker

[SauravMaheshkar]

Related Issue(s)	#1840
Has Unit Tests (y/n)	n
Documentation Included (y/n)	n

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Change Description

This PR adds a workflow for checking broken links in Markdown files

Rationale

Fixes #1840

[LeStarch]

Well, without surprise....this yielded many changes necessary.

[thomas-bc]

I think we may want to use

  - use-quiet-mode: 'yes'

Specify yes to only show errors in output.

https://github.com/gaurav-nelson/github-action-markdown-link-check?tab=readme-ov-file#custom-variables

[SauravMaheshkar]

How would you like to proceed with this:

A) incorporate all links fixes in this PR. (maybe even one squash'ed commit)
B) Merge this PR, and make subsequent PRs fixing the links.

Something else?

[thomas-bc]

I think we're going to want to go with A otherwise it's going to break other PRs and the main branch. We squash PRs as a single commit so no worries about making lots of small commits. I'll give you a hand as soon as I can

[thomas-bc]

Well, that was cumbersome, but that should do it!

A very few things was so old that I removed it.
Most were tracked down.
A couple in the CMake API I hardcoded, since the relative ../api/ links are dead in Markdown anyways, since api/ is generated a docgen-time

We might take a little while to review everything. Will get back to you once we got back to it.

[SauravMaheshkar]

Well, that was cumbersome, but that should do it!

A very few things was so old that I removed it. Most were tracked down. A couple in the CMake API I hardcoded, since the relative ../api/ links are dead in Markdown anyways, since api/ is generated a docgen-time

We might take a little while to review everything. Will get back to you once we got back to it.

Thanks for all the help @thomas-bc

[LeStarch]

Might need to rework with the rewritten Test API guide.

[thomas-bc]

@SauravMaheshkar would you be able to make the changes that were requested by LeStarch's review, as well as resolving the merge conflict? All changes from devel in docs/UsersGuide/dev/testAPI/user_guide.md should be taken in