docs/devel: update build system docs

configure is only doing compiler and host setup now, so adjust the
relevant documentation.  It is also possible to build emulators with
ninja directly if one is so inclined, so mention that as well.

The Python virtual environment set up is a new major task of configure
as well.  Mention it in the list of produced files, while leaving it
for a future patch to document how it works and how ``mkvenv ensure``
is used.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>