btrfs-progs: docs: move Interoperability to the first section

The page is not complete but mentions at least NFS, cgroups and
fsverity.

[ci skip]

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

Documentation/index.rst

@@ -30,6 +30,7 @@ is in the :doc:`manual pages<man-index>`.
    Glossary
    INSTALL
    Source-repositories
+   Interoperability
 
 .. raw:: html
 
@@ -103,5 +104,4 @@ for missing contents.
    :caption: TODO
 
    Quick-start
-   Interoperability
    trouble-index