docs: add HCPEtcdBackup implementation reference

Document the full HCPEtcdBackup integration including architecture,
backup/restore flows, configuration, credential handling, storage layout,
dependency chain (PRs #8139, #8010, #8017, #8040, #8114, enhancement #1945),
and troubleshooting guide.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Juan Manuel Parrilla Madrid <jparrill@redhat.com>

Author: jparrill