docs/devel: Add a single top-level header to MTTCG's doc

Without a single top-level header in the .rst file, the index ended
up linking to all of the top-level headers separately. Now the index
links to the top-level header at the beginning of the document and
any inner headers are correctly linked as sub-items in the index.

Signed-off-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210528123526.144065-1-luis.pires@eldorado.org.br>
Message-Id: <20210623102749.25686-3-alex.bennee@linaro.org>

docs/devel/multi-thread-tcg.rst

@@ -4,8 +4,9 @@
   This work is licensed under the terms of the GNU GPL, version 2 or
   later. See the COPYING file in the top-level directory.
 
-Introduction
-============
+==================
+Multi-threaded TCG
+==================
 
 This document outlines the design for multi-threaded TCG (a.k.a MTTCG)
 system-mode emulation. user-mode emulation has always mirrored the