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Let's hope this is not too annoying to users who build on outdated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

build.rs

@@ -97,9 +97,6 @@ fn detect_cfgs(target: &mut Target) {
             }
         }),
         ("small_main_stack", &has_small_stack),
-        ("use_prebuilt_docs", &|_| {
-            env_var("FISH_USE_PREBUILT_DOCS").is_some_and(|v| v == "TRUE")
-        }),
         ("using_cmake", &|_| {
             option_env!("FISH_CMAKE_BINARY_DIR").is_some()
         }),