Add llms.txt to texera.apache.org

### Feature Summary

[llms.txt](https://llmstxt.org/) is an emerging convention for exposing a site's documentation in a machine-readable index that LLM agents can consume — conceptually similar to a sitemap for crawlers, but aimed at agents.

Texera's documentation is hosted on texera.apache.org. Without an `llms.txt`, agents have to scan source code or the rendered site to answer questions about Texera. Publishing an `llms.txt` would give them a structured directory of the site's content, enabling things like:

- Answering "does Texera support X?" by reading the docs index instead of scanning source code.
- Serving as the foundation for a Texera agent knowledge base (similar to what Spark has done).
- A low-cost, structured entry point that we can grow as more content lands on apache.org.

Example from ng-zorro: https://ng.ant.design/llms.txt

### Proposed Solution or Design

1. Add `llms.txt` to [apache/incubator-texera-site](https://github.com/apache/incubator-texera-site) so it is served at `texera.apache.org/llms.txt`.
2. Add an `AGENTS.md` to that repo noting that `llms.txt` should be kept in sync when site content changes (updates are expected to be infrequent).
3. Gradually expand coverage as more content moves onto apache.org.

References:
- llms.txt spec: https://llmstxt.org/
- Example: https://ng.ant.design/llms.txt
- Site repo: https://github.com/apache/incubator-texera-site

### Impact / Priority

(P3) Low – nice to have, low maintenance cost.

### Affected Area

Other (apache/incubator-texera-site)