Add build script for llms.txt and markdown docs

[Azaya89]

Fixes #1730

Description

This PR adds a build script for generating llms.txt and the relevant markdown docs.

• Reworked https://github.com/panel-extensions/panel-material-ui/blob/main/scripts/build_llms_txt.py to generate hvPlot-focused markdown/llms.txt from selected doc/ content:
• include tutorials/, key ref/ pages, ref/plotting_options/, ref/api/manual/, and ref/api_compatibility/
• exclude doc/gallery/ and doc/user_guide/
• copy existing .md files and convert in-scope notebooks to markdown
• Updated pixi.toml docs tasks
• Also updated .github/workflows/docs.yaml build step

Unrelated

• Removed Streaming from doc/user_guide/index.md to clear the doc build warnings.

AI Disclosure

Tool & Model: GPT-5.3 Codex
Usage:

• I have tested all AI-generated content in my PR.
• I take responsibility for all AI-generated content in my PR.

Screenshot of generated llm.txt page

[maximlt]

How long does the _docs-markdown step take? Are we going to have to add this script to every repo or are we planning to upstream that to nbsite or some other place?

[Azaya89]

How long does the _docs-markdown step take?

It's actually surprisingly fast. Didn't extend the build time significantly in any noticeable way.

Are we going to have to add this script to every repo or are we planning to upstream that to nbsite or some other place?

We may have to add it to every repo because each repo will have different sets of docs it needs to convert to markdown.