bpf/docs: Summarize CI system and deny lists

This change adds a brief summary of the BPF continuous integration (CI)
to the BPF selftest documentation. The summary focuses not so much on
actual workings of the CI, as it is maintained outside of the
repository, but aims to document the few bits of it that are sourced
from this repository and that developers may want to adjust as part of
patch submissions: the BPF kernel configuration and the deny list
file(s).

Changelog:
- v1->v2:
  - use s390x instead of s390 for consistency

Signed-off-by: Daniel Müller <[email protected]>
Acked-by: David Vernet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Martin KaFai Lau <[email protected]>

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