bpf, doc: Link to submitting-patches.rst for general patch submission info

The link for patch submission information in general refers to index
page for "Working with the kernel development community" section of
kernel docs, whereas the link should have been
Documentation/process/submitting-patches.rst instead.

Fix it by replacing the index target with the appropriate doc.

Fixes: 54222838 ("bpf, doc: convert bpf_devel_QA.rst to use RST formatting")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230228074523.11493-3-bagasdotme@gmail.com