* ci(docs): RTD Sphinx warnings false successes
Use the same options on readthedocs.com that we use locally to fail on all warnings,
mostly to better catch stale cross references.
* docs(usage): Yet another stale Sphinx cross ref
I confirmed the previous RTD CI fix in the previous commit by pushing it to a draft PR
before pushing this change and the RTD action failed with the warning this commit
fixes. After pushing this commit, the RTD action succeeded.
* build(docs): Sphinx stale cross refs false success
I figured out why I kept getting warnings for broken Sphinx cross-refs *after* the
changes that caused them have already been merged, changes I know I ran `$ make docs`
for before pushing. The issue is that by default Sphinx only builds changed files for
faster iterations while editing, but it only catches broken cross-refs when it builds
files. So if changing, for example, a section name in one page that is referenced from
another page that you do *not* change, then the warning will be missed until something
changes that other page, such as a pull or rebase.
I considered adding a separate `./Makefile` target for incremental builds
in the inner loop of making changes. But I opted to just remove the `--write-all` CLI
option locally while editing in the inner loop, because the uncommitted change will
remind me to revert it and run a full rebuild before pushing.
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