docs: Add bpf/index to top level index

Recently bpf docs were converted to RST format.  The new files were not
added to the top level toctree.  This causes build system to emit a
warning of type

	WARNING: document isn't included in any toctree

Add bpf/index.rst to Documentation/index.rst
Signed-off-by: NTobin C. Harding <me@tobin.cc>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

Documentation/index.rst

@@ -90,6 +90,7 @@ needed).
    crypto/index
    filesystems/index
    vm/index
+   bpf/index
 
 Architecture-specific documentation
 -----------------------------------