docs: filesystems: convert befs.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 <[email protected]>
Link: https://lore.kernel.org/r/3e29ea6df6cd569021cfa953ccb8ed7dfc146f3d.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <[email protected]>
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index c948013..98de437f 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -52,6 +52,7 @@
    afs
    autofs
    autofs-mount-control
+   befs
    fuse
    overlayfs
    virtiofs