docs: filesystems: convert debugfs.txt to ReST

- Add a SPDX header;
- Use copyright symbol;
- Add a document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Use footnoote markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/42db8f9db17a5d8b619130815ae63d1615951d50.1581955849.git.mchehab+huawei@kernel.org


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