fix: drop mkdocs nav entries for notebooks the book never builds

[tschm]

Unblocks #538 (the rhiza v1.6.0 update), which fails book / build.

What's wrong

mkdocs.yml navigated to five notebook pages the book never builds:

- Notebooks:
    - Experiment1: notebooks/Experiment1.html
    ...

These have never resolved. Both paths are 404 on the live site right now:

• https://tschm.github.io/cs/notebooks/Experiment1.html → 404
• https://tschm.github.io/cs/marimo/notebooks/Experiment1.html → 404 (the path book/_toc.yml links to)

The book build has been saying so all along, in a warning nothing gated on:

[WARN] no marimo folder; skipping notebook export

rhiza-task exports notebooks from marimo_folder. This repo never sets it, so it
resolves to the CLI default docs/notebooks, which doesn't exist here — the notebooks
live under book/marimo/notebooks, which is named by source_folder. The export
no-ops, and the five nav entries dangle.

rhiza v1.6.0 adds a book-nav gate that checks every nav target against the built
site, which is what surfaced this:

[ERROR] nav target not in the built book: notebooks/Experiment1.html
...
failed  book-nav  5 of 12 nav target(s) missing from '_book'
        -- the site would publish a 404 in its own navigation

What this does

Removes the five entries, so the nav describes what the site actually publishes. The
remaining 7 nav targets all resolve. No other file changes.

What this deliberately does not do

Make the notebooks publish. That looks like a one-line marimo-folder setting, but it
isn't:

• Both the exporter (folder.glob("*.py")) and rhiza_marimo.yml's matrix
  (find -maxdepth 1 -name "*.py") glob every Python file with no notebook filter.
• book/marimo/notebooks also holds optimize.py and preamble.py, which are plain
  modules, not marimo notebooks, and carry no PEP 723 header. The matrix runs
  uv run --script on each match, so it would go from 0 jobs to 7 with 2 failures.
• Separating them means moving preamble.py off the notebooks' sys.path, and all
  five notebooks plus optimize.py do from preamble import …. It would also touch
  the ty extra-paths headers, the 1:1 table in docs/development/TEST_LAYOUT.md,
  scripts/check_test_layout.py, and source_folder.

Worth doing as its own change if those pages should be on the site. Filed separately.

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• mkdocs.yml

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