The help_sections.rs file was added to the tarball only as a quick hack.
There is a cyclic dependency between docs and fish:
"fish_indent" via "crates/build-man-pages" depends on "doc_src/".
So every "touch doc_src/foo.rst && ninja -Cbuild sphinx-docs"
re-builds fish.
In future "fish_indent" should not depend on "crates/build-man-pages".
Until then, a following commit wants to break this cyclic dependency
in a different way: we won't embed man pages (matching historical
behavior), which means that CMake builds won't need to run
sphinx-build.
But sphinx-build is also used for extracting help sections.
Also, the fix for #12082 will use help sections elsewhere in the code.
Prepare to remove the dependency on doc_src by committing the help
sections (we already do elsewhere).
