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>

1 files changed, 2 insertions, 1 deletions

diff --git a/docs/specs/index.rst b/docs/specs/index.rst
index 65e9663..ecc4389 100644
--- a/docs/specs/index.rst
+++ b/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.