It makes little sense to document the gallium envvars in multiple
places. Let's merge them all into the root envvar document.

This also moves GALLIUM_OVERRIDE_CPU_CAPS to toe root envvars doc, as
it's more generic than just LLVMpipe.

Reviewed-by: Yonggang Luo <luoyonggang@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19593>