docs: filesystems: convert nilfs2.txt to ReST

- Add a SPDX header;
- Add a document title;
- Adjust document title;
- Mark literal blocks as such;
- use :field: markup;
- 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/f7989ca501585f5990fffd2d365cfca4fe9fdd6f.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -70,9 +70,10 @@ Documentation for filesystem implementations.
    hfs
    hfsplus
    hpfs
+   fuse
    inotify
    isofs
-   fuse
+   nilfs2
    overlayfs
    virtiofs
    vfat