bpf, doc: rename txt files to rst files

This will cause them to get auto rendered, e.g. when viewing them on GitHub.
Followup patches will correct the content to be RST compliant.

Also adjust README.rst to point to the renamed files.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>