docs: filesystems: convert gfs2-uevents.txt to ReST

This document is almost in ReST format: all it needs is to have
the titles adjusted and add a SPDX header. In other words:

- Add a SPDX header;
- Add a document title;
- Adjust section titles;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Bob Peterson <rpeterso@redhat.com>
Link: https://lore.kernel.org/r/1d1c46b7e86bd0a18d9abbea0de0bc2be84e5e2b.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>