docs: add comprehensive Rust generator type mapping documentation

Created detailed documentation covering:
- Current type mappings from OpenAPI to Rust
- How oneOf, anyOf, and allOf are handled (or not)
- Alternative approaches that were considered
- Configuration options and their impact
- Known limitations and rationale for decisions
- Proposed changes in PR #21915 for true anyOf support

This documentation provides the context needed to understand both
the current implementation and the proposed improvements.

Author: timvw