markdownlint

[rrotter]

Done so far:

• stub out relevant docs in contributing.md
• add config file: .markdownlint.yml
  • disabled every rule we're violating that markdownlint doesn't know how to autocorrect w/ --fix flag
• disable linter at a few key points in the docs
• make the needed changes to ONE MARKDOWN FILE to make it pass the linter (appropriately chose docs/contributing.md)

To do:

• Wait for markdown spelling and formatting #418 to merge, rebase to master (I'll do this even if I never get to the rest of this); this is important because the rest of this WILL create changes that won't merge cleanly with that commit (or any major doc changes)
• Run markdownlint --fix '**/*.md', review the hundreds of changes
• commit
• Re-enable rules in .markdownlint.yml one at a time, run markdownlint '**/*.md', and either remediate the effected docs or dicide to keep the rule
• commit
• Possibly look at git options that prevent lint changes from breaking git blame. 99% sure this is possible.

Notes:
The main markdown linters I found are:

• markdownlint/markdownlint: ruby
• DavidAnson/markdownlint: Node.js

I'm using the latter because it appears to have more active maintenance including, critically, on the ruleset. I'm using markdownlint-cli rather than markdownlint-cli2 because there seems to be better support of the former in existing github actions.

I don't have any plans to work on this further anytime soon, so if someone finds this work valuable please feel free to pick it up and run with it.

[kdb424]

We should consider adding a make arg for this as well, and possibly (in a different PR) make a make command that should be run before committing to ensure that tests all pass, auto format code, ect, so there is less things to run before committing.