docs: refine ADR-0009 for Akrites CDP auth (CM-1333)

Align proposed approach with LF-owned Secrets Manager cross-account read
model, prune Alternatives to shared client_secret fallback only, switch
scopes to read:packages + read:advisories + read:maintainers, and fix
Mermaid diagram parse errors.

Signed-off-by: Joana Maia <jmaia@contractor.linuxfoundation.org>

Author: joanagmaia