qemu-doc: merge qemu-tech and qemu-doc

Merge what is left of qemu-tech into the main manual as an appendix.
Ultimately we should have a new internals manual built from docs/, and
then the "Translator Internals" parts of qemu-tech could move to docs/
as well.  The bits on limitation and features of CPU emulation should
remain in qemu-doc.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
