Add `MAINTAINERS.md` file and auto-generation machinery.

This change adds `MAINTAINERS.md` based on a single source-of-truth
YAML file, which also powers `.github/reviewer.json` to avoid modifying
both files separately (and to catch any PRs that doesn't edit both).

Did my best to backfill the historical list of maintainers based on a
mix of git commit history data and personal knowledge.

Fixes #14095.