Documentation: front page: use recommended heading adornments

Convert the Documentation front page to use the heading adornments
that are documented in doc-guide/sphinx.rst for document title and
chapters. I.e., convert most section headings to chapters.

This leaves "Indices and tables" as a chapter entry at the same level
as the other chapters.

The only visual difference from before to after is that the "Indices
and tables" heading is smaller and has more vertical whitespace
preceding it (although that may depend on the web browser being used).

Fixes: 0c7b4366 ("docs: Rewrite the front page")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/20230215005726.27320-1-rdunlap@infradead.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/index.rst

@@ -2,6 +2,7 @@
 
 .. _linux_doc:
 
+==============================
 The Linux Kernel documentation
 ==============================
 
@@ -13,7 +14,7 @@ documentation are welcome; join the linux-doc list at vger.kernel.org if
 you want to help out.
 
 Working with the development community
---------------------------------------
+======================================
 
 The essential guides for interacting with the kernel's development
 community and getting your work upstream.
@@ -29,7 +30,7 @@ community and getting your work upstream.
 
 
 Internal API manuals
---------------------
+====================
 
 Manuals for use by developers working to interface with the rest of the
 kernel.
@@ -43,7 +44,7 @@ kernel.
    Locking in the kernel <locking/index>
 
 Development tools and processes
--------------------------------
+===============================
 
 Various other manuals with useful information for all kernel developers.
 
@@ -62,7 +63,7 @@ Various other manuals with useful information for all kernel developers.
 
 
 User-oriented documentation
----------------------------
+===========================
 
 The following manuals are written for *users* of the kernel — those who are
 trying to get it to work optimally on a given system and application
@@ -81,7 +82,7 @@ See also: the `Linux man pages <https://www.kernel.org/doc/man-pages/>`_,
 which are kept separately from the kernel's own documentation.
 
 Firmware-related documentation
-------------------------------
+==============================
 The following holds information on the kernel's expectations regarding the
 platform firmwares.
 
@@ -93,7 +94,7 @@ platform firmwares.
 
 
 Architecture-specific documentation
------------------------------------
+===================================
 
 .. toctree::
    :maxdepth: 2
@@ -102,7 +103,7 @@ Architecture-specific documentation
 
 
 Other documentation
--------------------
+===================
 
 There are several unsorted documents that don't seem to fit on other parts
 of the documentation body, or may require some adjustments and/or conversion
@@ -115,7 +116,7 @@ to ReStructured Text format, or are simply too old.
 
 
 Translations
-------------
+============
 
 .. toctree::
    :maxdepth: 2