docs: filesystems: convert bfs.txt to ReST

- Add a SPDX header;
- Adjust document title;
- 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/93991bcc05e419368ee1e585c81057fb2c7c8d2b.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 98de437..f74e6b2 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -53,6 +53,7 @@
    autofs
    autofs-mount-control
    befs
+   bfs
    fuse
    overlayfs
    virtiofs