docs: add Sourcey-generated llms index

[IrvingOgaz]

Closes #34414

What changed

• adds a root llms.txt index for contributor and architecture documentation
• adds sourcey.llms.config.ts so the index is reproducible and reviewable
• maps eight existing documents without changing generator runtime behavior

Reproduction evidence

• source repository commit: 819adc170c499f0871a9b77c32d6f98b00f8e391
• Sourcey version: 3.6.5
• command: node node_modules/sourcey/dist/cli.js build --config sourcey.llms.config.ts --output <output-dir>
• generated pages: 8
• llms.txt SHA-256: 8B271CF4ED5FA8F3F33EFE2080A2E5A42428E5D0636A17333E5605D4C189CF13
• a clean second build produced the same hash

Audited entries: architecture, development, contributing, blueprints, Kubernetes Operator RFC, Control Center RFC, Entities RFC, and generator file-structure RFC. Every generated URL resolved to its corresponding index.html in the build output.

Validation

• git diff --check
• Prettier 3.9.6 check for both added files
• clean Sourcey rebuild with identical SHA-256
• documentation-only commits use [ci skip] as allowed by the contribution guide

Prepared with AI assistance. The generated summaries come from the repository's own Markdown via Sourcey; I personally reviewed the configuration, all eight entries, and the reproducible output.

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• All continuous integration tests are green (documentation-only commits intentionally use [ci skip])
• Tests are added where necessary (not applicable: generated documentation index)
• The JDL part is updated if necessary (not applicable)
• jhipster-online is updated if necessary (not applicable)
• Documentation is added/updated where necessary
• Coding Rules & Commit Guidelines as per our CONTRIBUTING.md document are followed
• If AI coding assistants were used to produce significant parts of this PR, it is disclosed in the description above and credited via a Co-authored-by: trailer in the commit(s)
• I have personally reviewed, understood, and tested the changes — including any AI-generated code