The `lgpl-docs` and documentation embedding step is now solely invoked
from `./gir/generator.py` in the CI, and does not need the embed/purge
build features anymore.
The `lgpl-docs` crate and all precompiled (easy to get outdated!)
docmentation files are removed in favour of being generated at runtime
by the improved generator, both during local development as well as in
the CI.
to_snake detects GLEGL and GLX11 as one word (contrary to GLWayland),
leading to no hyphen or underscore separating the two.
Fixes: 4cc57723 ("gl: Regenerate with separate EGL/Wayland/X11 -sys crates")
