docs: bpf/index.rst: Add ringbuf.rst

Fix the following warning

Documentation/bpf/ringbuf.rst: WARNING: document isn't
included in any toctree

By adding it to the index.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200718165107.625847-10-dwlsalmeida@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/bpf/index.rst

@@ -58,6 +58,14 @@ Testing and debugging BPF
    s390
 
 
+Other
+=====
+
+.. toctree::
+   :maxdepth: 1
+
+   ringbuf
+
 .. Links:
 .. _Documentation/networking/filter.rst: ../networking/filter.txt
 .. _man-pages: https://www.kernel.org/doc/man-pages/