[jonathan-taylor]

Summary

• No R in the docs workflow: drops r-base and libtirpc-dev from build_docs.yml, and rpy2 from the doc extra. No docs page runs R; glmnet_vs_R.md only mentions it in prose. The test extra keeps rpy2, since the tests compare with R. The R install took 22 of the 35 minutes in the last run.
• Two pages at a time: jupyter-book build --execute --execute-parallel 2. Both recent runs (40549481674 on main, 40552961629) were killed with exit 143 partway through the book, while mystmd had every page's kernel running at once. That most likely ran the runner out of memory.
• Memory in the log: the book step prints free -m each minute, so if it still fails we can see whether memory is the cause.
• No deploy from other branches: the deploy job now runs only on main. A run on another branch only builds, which is how this branch is tested (run 40555718660).
• README: the docs build no longer needs R.
• Changelog: a bullet under a new Unreleased section.

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