ext4: move ext4.txt into its own directory

Move Documentation/filesystems/ext4.txt into
Documentation/filesystems/ext4/ext4.rst in preparation for adding more
ext4 documentation.

Note that the documentation isn't in rst format yet, but as it's not
linked from anywhere it won't cause build errors.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>