* [DOCS] Improve reroute processor documentation structure and clarity
Add detailed explanation of document processing after rerouting, including:
- How documents are processed through the new target's ingest pipeline
- Support for chaining multiple reroutes across pipelines
- Automatic cycle detection and failure handling
Reorganize content with clear section headings for better scannability:
- Operating modes (explicit destination vs data stream mode)
- Pipeline execution behavior
- Document processing after rerouting
- Field updates
- Permissions
- Configuration options
- Examples
Improve readability throughout with clearer language and better formatting.
Fixes feedback from issue raised by @ivosh requesting clarification on
post-reroute document processing behavior.
