Convert "translator internals" docs to RST, move to devel manual

Our user-facing manual currently has a section "translator internals"
which has some high-level information about the design of the
TCG translator. This should really be in our new devel/ manual.
Convert it to RST format and move it there.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190607152827.18003-2-peter.maydell@linaro.org
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>