GHA: use pyspelling directly

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

.github/scripts/spellcheck.yaml

@@ -2,6 +2,7 @@
 #
 # SPDX-License-Identifier: curl
 #
+# Docs: https://facelessuser.github.io/pyspelling/configuration/
 # Docs: https://github.com/UnicornGlobal/spellcheck-github-actions
 matrix:
   - name: Markdown