docs: standardize book titles to === with overline

Documents within a Sphinx manual are separate files and therefore can use
different conventions for headings.  However, keeping some consistency is
useful so that included files are easy to get right.

This patch uses a standard heading format for book titles, so that it is
obvious when a file sits at the top level toctree of a book or man page.
The heading is irrelevant for man pages, but keep it consistent as well.

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

docs/index.rst

@@ -3,6 +3,7 @@
    You can adapt this file completely to your liking, but it should at least
    contain the root `toctree` directive.
 
+================================
 Welcome to QEMU's documentation!
 ================================
 

docs/system/qemu-block-drivers.rst

@@ -1,5 +1,6 @@
 :orphan:
 
+============================
 QEMU block drivers reference
 ============================
 

docs/system/qemu-cpu-models.rst

@@ -1,5 +1,6 @@
 :orphan:
 
+==================================
 QEMU / KVM CPU model configuration
 ==================================
 

docs/system/qemu-manpage.rst

@@ -6,8 +6,9 @@
    parts of the documentation that go in the manpage as well as the
    HTML manual.
 
-Title
-=====
+=======================
+QEMU User Documentation
+=======================
 
 Synopsis
 --------

docs/tools/qemu-img.rst

@@ -1,3 +1,4 @@
+=======================
 QEMU disk image utility
 =======================
 

docs/tools/qemu-nbd.rst

@@ -1,3 +1,4 @@
+=====================================
 QEMU Disk Network Block Device Server
 =====================================
 

docs/tools/qemu-pr-helper.rst

@@ -1,3 +1,4 @@
+==================================
 QEMU persistent reservation helper
 ==================================
 

docs/tools/qemu-storage-daemon.rst

@@ -1,3 +1,4 @@
+===================
 QEMU Storage Daemon
 ===================
 

docs/tools/qemu-trace-stap.rst

@@ -1,3 +1,4 @@
+=========================
 QEMU SystemTap trace tool
 =========================