[ericof]

The documentation was scaffolded from an earlier version of the cookieplone-templates docs starter and had drifted from it. It still carried Read the Docs configuration with an unfilled MY_READTHEDOCS_PROJECT_SLUG placeholder, although the docs are built and published to GitHub Pages by docs-deploy.yml.

This PR brings the documentation up to the current standard:

• Toolchain: docs/ follows the current docs starter (pyproject.toml, Makefile, conf.py, Vale configuration), and the docs build runs on Python 3.14 like the package.
• Read the Docs removed: .readthedocs.yaml, the rtd-* Makefile targets, the README section and the CI path filter are gone. The OpenSearch, sitemap and robots.txt URLs now point to the GitHub Pages site.
• Content: the pages were revised to the Plone documentation style. They are organized by Diátaxis section, follow Vale and use MyST markup, and they cover the HTML and Markdown input formats across the Python API, the CLI and the HTTP service.

The docs jobs in main.yml (docs and docs-deploy) now run on Python 3.14, while the other jobs keep the 3.12 default from config.yml. The theme repository links and the edit path in conf.py pointed at a wrong repository name, collectivehtml2blocksdocumentation; they now point at collective/collective.html2blocks. The CLI and HTTP service details moved from the how-to pages into new reference pages, and the how-to pages link to them.

Closes #43