Add new model type integration guide (#8779)

* Add new model type integration guide

Comprehensive documentation covering all steps required to integrate
a new model type into InvokeAI, including:

- Backend: Model manager, configs, loaders, invocations, sampling
- Frontend: Graph building, state management, parameter recall
- Metadata, starter models, and optional features (ControlNet, LoRA, IP-Adapter)

Uses FLUX.1, FLUX.2 Klein, SD3, SDXL, and Z-Image as reference implementations.

* docs: improve new model integration guide

- Move document to docs/contributing/ directory
- Fix broken TOC links by replacing '&' with 'and' in headings
- Add code example for text encoder config (section 2.4)
- Add text encoder loader example (new section 3.3)
- Expand text encoder invocation to show full conditioning flow (section 4.2)

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Co-authored-by: Lincoln Stein <lincoln.stein@gmail.com>

Author: Pfannkuchensack