Now, `ninja docs-libc-html` will re-run docgen.
Previously, we would run docgen offline, and commit the result.
Now we no longer need to do that; docgen is invoked from the
dependencies of the `docs-libc-html` target on demand. This
commit removes the dynamically generated .rst files (keeping
the static ones that haven't been converted to docgen), and
fixes up some mistakes I failed to cleanup recently since I
didn't have such automation in place to catch such bugs.
