docs: fix broken relative links across documentation

Fixes 46 broken relative links across 20 documentation files in docs/.

Changes:
- Added ../ prefix for root files (README.md, AGENTS.md)
- Removed ../ prefix for sibling docs files
- Removed dead links to non-existent files
- Fixed subdirectory paths

Based on PR #574 by chrisvaillancourt.

Co-Authored-By: Chris Vaillancourt <chrisvaillancourt@users.noreply.github.com>

docs/AGENT_MAIL.md

@@ -511,8 +511,6 @@ jobs:
 
 - [ADR 002: Agent Mail Integration](adr/002-agent-mail-integration.md)
 - [Agent Mail Repository](https://github.com/Dicklesworthstone/mcp_agent_mail)
-- [Integration Status](../AGENT_MAIL_INTEGRATION_STATUS.md)
-- [Latency Benchmark Results](../latency_results.md)
 - [Python Agent Example](../examples/python-agent/)
 
 ## Contributing