feat: overhaul call error documentation with fault classification and troubleshooting guide (#988)

Rewrites call-ended-reason.mdx to add error prefix explanation (vapifault/providerfault/pipeline-error),
quick diagnosis table, and ~40 missing error codes (billing, transfers, SIP, infrastructure).
Removes 5 phantom values that don't exist in the codebase.

Creates a new troubleshoot-call-errors.mdx guide organized by caller-experienced symptoms
(6 impact buckets) so customers can quickly diagnose what went wrong.

Adds cross-links from debugging.mdx and updates docs.yml navigation.

Author: dhruva-reddy