docs: filesystems: convert btrfs.txt to ReST

Just trivial changes:

- Add a SPDX header;
- Add it to filesystems/index.rst.

While here, adjust document title, just to make it use the same
style of the other docs.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: David Sterba <dsterba@suse.com>
Link: https://lore.kernel.org/r/1ef76da4ac24a9a6f6187723554733c702ea19ae.1581955849.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>