docs: filesystems: convert seq_file.txt to ReST

- Add a SPDX header;
- Add a document title;
- Adjust section titles;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/f950a0a56178ee05872ae2a2711a04d7af8ebb24.1588021877.git.mchehab+huawei@kernel.org


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