Remove man/HTML docs from tarball, require Sphinx instead

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Let's hope this is not too annoying to users who build on outdated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

README.rst

@@ -157,7 +157,7 @@ In addition to the normal CMake build options (like ``CMAKE_INSTALL_PREFIX``), f
 - Rust_COMPILER=path - the path to rustc. If not set, cmake will check $PATH and ~/.cargo/bin
 - Rust_CARGO=path - the path to cargo. If not set, cmake will check $PATH and ~/.cargo/bin
 - Rust_CARGO_TARGET=target - the target to pass to cargo. Set this for cross-compilation.
-- BUILD_DOCS=ON|OFF - whether to build the documentation. This is automatically set to OFF when Sphinx isn't installed.
+- WITH_DOCS=ON|OFF - whether to build the documentation. By default, this is ON when Sphinx is installed.
 - FISH_USE_SYSTEM_PCRE2=ON|OFF - whether to use an installed pcre2. This is normally autodetected.
 - MAC_CODESIGN_ID=String|OFF - the codesign ID to use on Mac, or "OFF" to disable codesigning.
 - WITH_GETTEXT=ON|OFF - whether to include translations.