Advantages of prebuilt docs:
- convenient for users who compile the tarball
- convenient for packagers who don't have sphinx-build packaged
(but packaging/installing that should be easy nowadays?
see https://github.com/fish-shell/fish-shell/issues/12052#issuecomment-3520336984)
Disadvantages:
- Extra build stage / code path
- Users who switch from building from tarball to building from source
might be surprised to lose docs.
- People put the [tarballs into Git repositories](https://salsa.debian.org/debian/fish), which seems weird.
Remove the tarball.
Let's hope this is not too annoying to users who build on outdated
distros that don't have sphinx -- but those users can probably use
our static builds, skipping all compilation.
To avoid breaking packagers who use `-DBUILD_DOCS=OFF` (which still
installs prebuilt docs), rename the option.
Closes#12088
135fc73191