docs: filesystems: convert ecryptfs.txt to ReST

docs: filesystems: convert ecryptfs.txt to ReST

- Add a SPDX header;
- Add a document title;
- use :field: markup;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Tyler Hicks <code@tyhicks.com>
Link: https://lore.kernel.org/r/6e13841ebd00c8d988027115c75c58821bb41a0c.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>