docs: filesystems: convert f2fs.txt to ReST

- Add a SPDX header;
- Adjust document and section titles;
- 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>
Link: https://lore.kernel.org/r/8dd156320b0c015dec6d3f848d03ea057042a15b.1581955849.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -64,6 +64,7 @@ Documentation for filesystem implementations.
    erofs
    ext2
    ext3
+   f2fs
    fuse
    overlayfs
    virtiofs