Implement "markdownlint" for linting of the docs

[tarkatronic]

Feature Request

Is your feature request related to a problem? Please describe.

No problem. But more linters and more consistency == more good!

Describe the solution you'd like

There are apparently two markdownlint tools available; one in Ruby (https://github.com/markdownlint/markdownlint), and one in NodeJS (https://github.com/igorshubovych/markdownlint-cli). There is also a (no longer active) pymarkdownlint (https://github.com/jorisroovers/pymarkdownlint).

I'm not really sure the advantages/disadvantages of each. Except that the Python one is incomplete and abandoned. I think it will take some experimentation and perhaps somebody with a strong opinion to decide which to use.

Each has a .pre-commit-hooks.yaml, meaning we can use it with our existing pre-commit toolchain. I guess it's just a matter of, do we want to introduce Ruby or JS to the stack?

Describe alternatives you've considered

The alternative is that I continue manually linting things when I edit the docs, and that sucks. 😄

Teachability, Documentation, Adoption, Migration Strategy

For migration, we can do one fell swoop to get all the docs in compliance with the linter. And then adding it to the pre-commit config will be easy enough. There shouldn't need to be any additional teaching there. The one complication we will have is that I don't know how easily we can integrate this with tox. Might need some thought on that one...