docs/bpf: Update bpf development Q/A file

docs/bpf: Update bpf development Q/A file

bpf now has its own mailing list [email protected].
Update the bpf_devel_QA.rst file to reflect this.

Also llvm has switch to github with llvm and clang
in the same repo https://github.com/llvm/llvm-project.git.
Update the QA file with newer build instructions.

Signed-off-by: Yonghong Song <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: Song Liu <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]