docs: add table of contents to QAPI references

docs: add table of contents to QAPI references

The QAPI reference docs for the guest agent, storage daemon and QMP are
all rather long and hard to navigate unless you already know the name of
the command and can do full text search for it.

A table of contents in each doc will help people locate stuff much more
easily.

Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>