gh-41312: Explain how to disable generating targets for docbuilding at `pip install` step
Follow-up on https://github.com/sagemath/sage/pull/41191. In the
"background information" section below there's a guide how to do the
equivalent with `meson`, but having more can't hurt (hopefully).
### :memo: Checklist
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- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/41312
Reported by: user202729
Reviewer(s):
Committer
Release Manager