fix: #3613484 Publish the documentation with mkdocs and unblock apt in CI

Follow-up on #3613484, on the same issue, after MR !6 (merged) merged.

The docs were not published

The pages job that ships in the GitLab templates this project includes is gated on mkdocs.yml and builds with pip install mkdocs-material then mkdocs build. The gitlab_templates project itself has moved to zensical, but the templates ref resolved here has not, so a zensical.toml alone produced no pages job at all and https://project.pages.drupalcode.org/ai_agent_modes/ served nothing.

This ships mkdocs.yml with the same title, nav, repo and edit_uri, and removes zensical.toml so there is one source of truth. md_in_html is enabled so the HTML5 video players in the pages survive the build.

Verified with mkdocs build --strict in a python:3.12-slim container: all five pages build, the three pages with players keep them, and every image and video URL resolves.

CI: apt was broken on 1.0.x

Both functional-testing jobs failed on the post-merge 1.0.x pipeline with:

E: Failed to fetch https://deb.nodesource.com/node_20.x/dists/nodistro/InRelease  403  Forbidden
E: The repository 'https://deb.nodesource.com/node_20.x nodistro InRelease' is not signed.
Failed to install browsers
Error: Installation process exited with code: 100

The image carries a nodesource 20.x apt list whose InRelease now returns 403, so any later apt-get update fails, including the one inside playwright install --with-deps. This restores the removal of that list; it was dropped when the Node install block was rewritten to use the official tarball. The tarball install itself is confirmed working (Installing node-v22.23.1-linux-x64.tar.gz, version guard passing).

No module code, configuration or test changes.

Checkpoints

• File an issue
• Addition/Change/Update/Fix
• Reviewed by a human
• Code review by maintainers