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: Tobin C. Harding <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

Documentation/index.rst

···
    crypto/index
    filesystems/index
    vm/index
+   bpf/index
 
 Architecture-specific documentation
 -----------------------------------