docs: create filesystem internal section

Create a new top-level section for documentation of filesystem usage,
on-disk format information, and anything else.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Documentation/filesystems/ext4/index.rst

@@ -1,8 +1,8 @@
 .. SPDX-License-Identifier: GPL-2.0
 
-====
-ext4
-====
+===============
+ext4 Filesystem
+===============
 
 General usage and on-disk artifacts writen by ext4.  More documentation may
 be ported from the wiki as time permits.  This should be considered the