Add more pull request tests to enforce standards and quality

There are a number of tools that can be setup to run basic checks or tests.

Listing here some improvements we could add:

- link checkers: lychee or markdown-link-check
- spell checkers: codespell or typos
- linters: markdownlint, yamllint, actionlint, zizmor, shellcheck, ruff, oxipng, bandit and more...
- pre-commit which can run both locally and with actions on GitHub

The issue names no files, tests, or workflow entry points. Start by inventorying the existing pull request checks and decide which of the listed link checkers, spell checkers, linters, or pre-commit integrations are in scope; done means the selected checks run for pull requests and enforce the agreed standards.

Written by the indexing model from the issue text.