Documentation: gfs2: Consolidate GFS2 docs into its own subdirectory

Documentation for GFS2 is scattered in three docs that are in
Documentation/filesystems/ directory. As these docs are standing out as
a group, move them into separate gfs2/ subdirectory.

Reviewed-by: Randy Dunlap <[e-mail]>
Signed-off-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Andreas Gruenbacher <[e-mail]>

Documentation/filesystems/gfs2.rst Documentation/filesystems/gfs2/index.rst

···
 Global File System 2
 ====================
 
+Overview
+========
+
 GFS2 is a cluster file system. It allows a cluster of computers to
 simultaneously use a block device that is shared between them (with FC,
 iSCSI, NBD, etc).  GFS2 reads and writes to the block device like a local
···
   gfs2_convert		to convert a gfs filesystem to GFS2 in-place
   mkfs.gfs2		to make a filesystem
   ============		=============================================
+
+Implementation Notes
+====================
+
+.. toctree::
+   :maxdepth: 1
+
+   glocks
+   uevents

Documentation/filesystems/index.rst

···
    ext3
    ext4/index
    f2fs
-   gfs2
-   gfs2-uevents
-   gfs2-glocks
+   gfs2/index
    hfs
    hfsplus
    hpfs

MAINTAINERS

···
 S:	Supported
 B:	https://bugzilla.kernel.org/enter_bug.cgi?product=File%20System&component=gfs2
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/gfs2/linux-gfs2.git
-F:	Documentation/filesystems/gfs2*
+F:	Documentation/filesystems/gfs2/
 F:	fs/gfs2/
 F:	include/uapi/linux/gfs2_ondisk.h