To avoid depending on Docker Hub, an Docker image and a GitHub Action.
Also to simplify running this check on a local machine.
Pending question if Dependabot and Mend/Renovate will automatically pick
up `requirements-docs.txt`.
Also:
- enable parallel spellchecking. (also to win back the time lost with
installing components directly from Debian and pip.)
- pin `pyspelling`.
- link to official `pyspelling` docs.
Closes#18736
