Incorrect documentation paths referenced in llms.txt

[GBLightning]

Description
Several documentation links in llms.txt point to non-existing or outdated paths.

To Reproduce
Open llms.txt, scroll to the Documentation section at the end of the file, and try opening the listed links.

Expected behavior
All links should lead to existing documentation files.

Additional context

1. https://github.com/google/adk-docs/blob/main/docs/agents/models.md - was removed because it was only a template.
2. https://github.com/google/adk-docs/blob/main/docs/deploy/agent-engine.md - was separated into several .md files - docs/deploy/agent-engine/asp.md, docs/deploy/agent-engine/deploy.md, docs/deploy/agent-engine/index.md, docs/deploy/agent-engine/test.md.
3. https://github.com/google/adk-docs/blob/main/docs/get-started/testing.md - was renamed (C099) to docs/runtime/api-server.md
4. https://github.com/google/adk-docs/blob/main/docs/streaming/custom-streaming-ws.md - was removed.
5. https://github.com/google/adk-docs/blob/main/docs/streaming/custom-streaming.md - was removed.
6. https://github.com/google/adk-docs/blob/main/docs/tools/authentication.md - was moved (R100) to docs/tools-custom/authentication.md
7. https://github.com/google/adk-docs/blob/main/docs/tools/built-in-tools.md - was reorganized and separated into several files.
8. https://github.com/google/adk-docs/blob/main/docs/tools/function-tools.md - was moved (R099) to docs/tools-custom/function-tools.md
9. https://github.com/google/adk-docs/blob/main/docs/tools/google-cloud-tools.md - was moved (R074) to docs/tools/google-cloud/application-integration.md
10. https://github.com/google/adk-docs/blob/main/docs/tools/mcp-tools.md - was moved (R100) to docs/tools-custom/mcp-tools.md
11. https://github.com/google/adk-docs/blob/main/docs/tools/openapi-tools.md - was moved (R100) to docs/tools/third-party/openapi-tools.md
12. https://github.com/google/adk-docs/blob/main/docs/tools/third-party-tools.md - was moved (R100) to docs/tools/third-party/index.md