docs: serve an llms.txt index of the documentation

[ghostiee-11]

LLM assistants now handle a large share of the "which library should I use for this" questions that used to go to search, and they read a site far better when it publishes a curated index than when they have to crawl it. llms.txt is the convention for that. marimo and Vizro both serve one. No HoloViz site does.

This adds docs/llms.txt.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.37%. Comparing base (733acbb) to head (2eafabc).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1939      +/-   ##
==========================================
+ Coverage   72.32%   72.37%   +0.04%     
==========================================
  Files         203      204       +1     
  Lines       35666    35714      +48     
==========================================
+ Hits        25796    25848      +52     
+ Misses       9870     9866       -4     

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[ahuang11]

Let's wait for https://github.com/holoviz/hvplot/pull/1732/changes to settle

[ghostiee-11]

Okay!!

[ahuang11]

Do you have an example artifact I can look at

[ghostiee-11]

lumen-llms.txt

[ahuang11]

> Lumen is an open-source and extensible agent framework for chatting with data and for retrieval augmented generation. It turns natural language into SQL, data transformation pipelines, visualizations and dashboards, and every step stays inspectable, editable and reproducible.

Lumen is built on Panel and the wider HoloViz stack. Its declarative data model means anything the language model produces can be serialized to YAML, reopened in a notebook, or composed into a dashboard.

Install with `pip install 'lumen[ai-openai]'` and start with `lumen-ai serve data.csv`.

Is this necessary for the LLM to know? And is it named lumen-llms.txt by you, or should it be named llms.txt?

These markdown files should help the LLM develop (think of it like a CLAUDE.md file).
For ref: https://panel-material-ui.holoviz.org/llms.txt

[ahuang11]

Let's wait until holoviz-dev/nbsite#361 is merged