Add Markdownlint to CI (and ignore all rules for now)

[SeanKilleen]

The first step in markdown linting per #5460.

Summary of the changes (Less than 80 chars)

• Adds Markdownlint runner to GitHub Actions
• Ignores all violations now so that we can begin to address them individually (avoids conflicts & large PRs, which stopped this effort previously).

[SeanKilleen]

@glen-84 or others -- please approve this workflow when you get the chance so we can validate that it runs. 👍

The next step will be to actually begin remediating errors -- this step will free us up to parallelize that across multiple contributors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4196aba) 70.65% compared to head (fb03673) 70.69%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6842      +/-   ##
==========================================
+ Coverage   70.65%   70.69%   +0.03%     
==========================================
  Files        2499     2499              
  Lines      126167   126167              
==========================================
+ Hits        89139    89189      +50     
+ Misses      37028    36978      -50     

Flag	Coverage Δ
unittests	70.69% <ø> (+0.03%)	⬆️

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[glen-84]

Thanks Sean. Let's wait for Michael to approve, and then I can probably assist with the fixes.

[SeanKilleen]

@glen-84 I'm happy to tackle the fixes. This is just the first step in doing it methodically.

@michaelstaib, whenever you're able to approve this CI job to execute, please do. Happy to contribute again :)

[michaelstaib]

Thanks @SeanKilleen for the help on this!