docs: link related technology integrations

[rlippmann]

Summary

Improves navigation between generic enforcement-point examples and related technology-specific integrations.

This PR adds cross-links so readers can move naturally from a generic enforcement-point example to concrete implementations built on specific frameworks, runtimes, and integration surfaces.

Why

The repository now contains two complementary layers:

• Generic enforcement-point examples that teach the architectural pattern.
• Technology-specific integrations that demonstrate the same pattern in real runtimes.

The examples were already present, but several relationships were difficult to discover because the READMEs were not linked together.

This PR improves discoverability without changing repository structure or adding new examples.

Changes

Top-level catalog

• Added links from enforcement-point catalog entries to the corresponding enforcement-point README pages.

Gateway middleware

Added links from gateway middleware documentation to:

• LiteLLM Proxy reference integration

Prompt construction

Added links to:

• Python LiteLLM examples
• Open WebUI pipe integration
• TypeScript Next.js starter applications

Schema selection

Added links to:

• Ollama structured output example
• LiteLLM response_format example
• Vercel AI SDK generateObject example

Checkpoint continuation

Added links to:

• FastAPI integration
• TypeScript Node starter
• TypeScript Next.js starter

Execution authorization

Added links to:

• TypeScript Node starter companion

Retrieval filtering

Added links to:

• ChromaDB retrieval-filtering example

Scope

This is a documentation-only navigation improvement.

No:

• runtime behavior changes
• example changes
• test changes
• validation script changes
• repository restructuring

are included.

Notes

The goal is to make the repository hierarchy more explicit:

Generic enforcement point
↓
Technology-specific implementation

The PR intentionally uses links rather than moving files or introducing duplicate examples.

Remaining Follow-ups

Potential future improvements:

• Add a dedicated README for schema_selection/litellm_response_format
• Consider language-neutral enforcement-point landing pages if cross-language navigation becomes more important

Neither is required for this PR.

Validation

Documentation-only changes.