docs: standardize directory index to --- with overline

Use a standard heading format for the index.rst file in a directory.
Using overlines makes it clear that individual documents can use e.g.
=== for chapter titles and --- for section titles, as suggested in the
Linux kernel guidelines[1].  They could do it anyway, because documents
included in a toctree are parsed separately and therefore are not tied
to the same conventions for headings.  However, keeping some consistency is
useful since sometimes files are included from multiple places.

[1] https://www.kernel.org/doc/html/latest/doc-guide/sphinx.html

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/about/index.rst

@@ -1,5 +1,6 @@
+----------
 About QEMU
-==========
+----------
 
 QEMU is a generic and open source machine emulator and virtualizer.
 

docs/devel/index.rst

@@ -1,5 +1,6 @@
+---------------------
 Developer Information
-=====================
+---------------------
 
 This section of the manual documents various parts of the internals of QEMU.
 You only need to read it if you are interested in reading or

docs/interop/index.rst

@@ -1,5 +1,6 @@
+------------------------------------------------
 System Emulation Management and Interoperability
-================================================
+------------------------------------------------
 
 This section of the manual contains documents and specifications that
 are useful for making QEMU interoperate with other software.

docs/specs/index.rst

@@ -1,5 +1,6 @@
+----------------------------------------------
 System Emulation Guest Hardware Specifications
-==============================================
+----------------------------------------------
 
 This section of the manual contains specifications of
 guest hardware that is specific to QEMU.

docs/system/index.rst

@@ -1,5 +1,6 @@
+----------------
 System Emulation
-================
+----------------
 
 This section of the manual is the overall guide for users using QEMU
 for full system emulation (as opposed to user-mode emulation).

docs/tools/index.rst

@@ -1,5 +1,6 @@
+-----
 Tools
-=====
+-----
 
 This section of the manual documents QEMU's "tools": its
 command line utilities and other standalone programs.

docs/user/index.rst

@@ -1,5 +1,6 @@
+-------------------
 User Mode Emulation
-===================
+-------------------
 
 This section of the manual is the overall guide for users using QEMU
 for user-mode emulation.  In this mode, QEMU can launch