docs: filesystems: convert sysfs.txt to ReST

- Add a SPDX header;
- Add a document title;
- Adjust document and section titles;
- use :field: markup;
- 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/5c480dcb467315b5df6e25372a65e473b585c36d.1581955849.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -87,5 +87,6 @@ Documentation for filesystem implementations.
    relay
    romfs
    squashfs
+   sysfs
    virtiofs
    vfat