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Let's hope this is not too annoying to users who build on outdated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

.github/workflows/release.yml

@@ -172,7 +172,7 @@ jobs:
           echo "$MACOS_NOTARIZE_JSON" >/tmp/notarize.json
           ./build_tools/make_macos_pkg.sh -s -f /tmp/app.p12 \
             -i /tmp/installer.p12 -p "$MAC_CODESIGN_PASSWORD" \
-            -n -j /tmp/notarize.json -- -c "-DBUILD_DOCS=ON"
+            -n -j /tmp/notarize.json -- -c "-DWITH_DOCS=ON"
           version=$(git describe)
           [ -f "${FISH_ARTEFACT_PATH}/fish-$version.app.zip" ]
           [ -f "${FISH_ARTEFACT_PATH}/fish-$version.pkg" ]