docs: filesystems: convert inotify.txt to ReST

- Add a SPDX header;
- Add a document title;
- Adjust document title;
- Fix list markups;
- Some whitespace fixes and new line breaks;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/8f846843ecf1914988feb4d001e3a53d27dc1a65.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>