ext4: convert ext4.rst to restructuredtext format

Convert the existing ext4 documentation into rst format and link it in
with the rest of the kernel documentation.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Documentation/filesystems/ext4/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+====
+ext4
+====
+
+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
+canonical source of information as the details here have been reviewed by
+the ext4 community.
+
+.. toctree::
+   :maxdepth: 5
+   :numbered:
+
+   ext4

Documentation/filesystems/index.rst

@@ -2,6 +2,14 @@
 Linux Filesystems API
 =====================
 
+The documentation in this section are provided by specific filesystem
+subprojects.
+
+.. toctree::
+   :maxdepth: 2
+
+   ext4/index
+
 The Linux VFS
 =============