move lint actions to their own job, add more checks, fix linting issues

[finn-block]

• an external contributor snuck some new dependencies in without updating package-lock.json, that sort of violation will now be caught in CI.
• another external contributor helped us follow the GitHub markdown accessibility guidelines, but didn't add a CI action to ensure we keep up to date. This adds markdown linting, and most violations have been fixed
  • I decided to disable the line length rule. It's 2023 and screens are somewhat wider than 80 characters. Maybe in the future we'll turn it on with a higher limit

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[codecov-commenter]

Codecov Report

Merging #85 (375e86b) into main (eee2559) will increase coverage by 2.27%.
Report is 2 commits behind head on main.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
+ Coverage   80.62%   82.90%   +2.27%     
==========================================
  Files          15       15              
  Lines         986     1000      +14     
  Branches      100      140      +40     
==========================================
+ Hits          795      829      +34     
+ Misses        191      171      -20     

see 3 files with indirect coverage changes