pipeline: router: add conditional routing documentation

Add comprehensive documentation for both tag-based routing and
the new conditional routing feature introduced in Fluent Bit 4.0.

- Document tag-based routing with basic matching, wildcards, and regex
- Document conditional routing with per-record evaluation
- Include comparison operators and condition syntax
- Add examples for routing by severity, service name, and regex patterns
- Provide guidance on choosing between routing approaches

Fixes #2199.

Signed-off-by: Eric D. Schabell <[email protected]>

Author: eschabell