docs: comprehensive log collector documentation

## Summary PR #13354 added initial documentation for collected log files (`docs/collected-logs.md`). This issue tracks the follow-up work to create comprehensive documentation for the full log collection system. ## Scope - Document all log collectors in `sdcm/logcollector.py` (what they collect, when, from where) - Document per-backend differences in log availability - Document how to add new log files to collection - Document log rotation and size limits - Add examples of using logs for debugging common issues - Document the relationship between Argus log links and S3 storage ## Context The initial `docs/collected-logs.md` covers the archive structure and file listing. This issue is for deeper documentation of the collector internals and usage patterns.