Description of Change

Use markdownlint to lint the docs and help keep them tidier going forward. markdownlint from DavidAnson/markdownlint, not to be confused with the markdownlint/markdownlint repo.

It's actively maintained, highly configurable, has integration with editors like VS Code, and other tooling support like GitHub problem matchers.

I've been making PRs to clean up the markdown, and the configuration in this PR is tailored to run cleanly with the current state of the docs. A couple of the rules off in this config may want to be turned on in the future (for example, no-duplicate-header - duplicate headers cause problems for linking since they're multiple places on the page), but I think it's a reasonable compromise for now. Line length is disabled for now; although the style guide says wrap at 80 characters, there's a lot of violations in the docs currently, and I don't want to wrestle with that.

PR also drops script/check-trailing-whitespace.py since markdownlint can provide that functionality. I don't love needing the second config file to get the Git hooks functionality, but I didn't want to switch on autofixing more complex rules, and they didn't want to take a PR to specify rules to fix on the CLI, so it was the only way to be a drop-in replacement.

The autofix config can be expanded to include rules like blank lines, but for the initial PR I've aimed to be no changes from the current behavior.

Runs clean, but some changes slipped in before I opened this PR, so #26793 needs to be merged first to unblock this.

Checklist

• PR description included and stakeholders cc'd

Release Notes

Notes: none