docs: name unreadable database certificate files

[indykish]

Summary

• Document that unreadable PostgreSQL certificate files now identify the
  responsible database setting.
• Document that malformed inline certificate data is redacted from errors.

Verification

• make lint: documentation checks passed (22 tests OK).
• The remaining OpenAPI drift failure was reproduced unchanged on a clean
  origin/main worktree and is unrelated to this changelog-only diff.

Companion code PR: agentsfleet/agentsfleet#642

Greptile Summary

Documents improved PostgreSQL certificate error reporting and redaction behavior.

• Adds an Aug 31 changelog entry explaining that unreadable certificate errors identify the database variable, canonical parameter, and path.
• Notes that malformed inline certificate data is redacted from errors.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
changelog.mdx	Adds a well-formed changelog entry describing PostgreSQL certificate diagnostics and sensitive inline-data redaction.

_{Reviews (2): Last reviewed commit: "Merge remote-tracking branch 'origin/mai..." | Re-trigger Greptile}