Add data retention configuration and cleanup functionality

- Introduced a new `[Data_Retention]` section in `config.ini.example` to manage retention periods for various database tables, including packet stream, daily stats, and observed paths.
- Updated `mkdocs.yml` and `configuration.md` to include documentation for the new data retention settings.
- Implemented data retention cleanup methods in `mesh_graph.py`, `repeater_manager.py`, and `scheduler.py` to enforce retention policies automatically.
- Enhanced the web viewer's data cleanup logic to utilize the new retention settings, ensuring efficient database management.

mkdocs.yml

@@ -56,6 +56,7 @@ nav:
     - Service: service-installation.md
   - Configuration:
     - Overview: configuration.md
+    - Data retention: data-retention.md
     - Path Command: path-command-config.md
     - Config validation: config-validation.md
   - Web Viewer: web-viewer.md