doc: Rename user-manual -> bitbake-user-manual

This manual gets combined with other manuals and in that context, it helps
a lot if its seen as the Bitbake User Manual. Renames are a pain but
this is worthwhile so that other docs can correctly be combined with this
one. This also clarifies things like google search results which is helpful.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>