Generate .md files for docs HTML pages

[marwie]

Related issue: 31933

Description

This PR adds a plugin to JSDoc to produce markdown files alongside the existing .html documentation pages and allows to append .md to docs URL hashes to open the markdown file. These files can then be used by LLMs to lookup information.

Example

• https://uploads.needle.tools/three/20260102-md/docs/#AnimationAction (Regular docs page)
• https://uploads.needle.tools/three/20260102-md/docs/#AnimationAction.md (Markdown version)

Open questions

1. Is this something three.js is even interested in since it adds some maintenance overhead?
2. Currently the JSDoc plugin is vibecoded (at first I tried using jsdoc-to-markdown but wasn't happy with the result). Maybe the existing docs template could also produce the markdown file in the same plugin? Or should the HTML output be converted to markdown instead?
3. What's missing is a llms.txt that contains links to the individual markdown files (similar to svelte llms.txt or stripe's llms.txt - the Stripe documentation works the same way https://docs.stripe.com/building-with-llms#llmstxt whereas Svelte does append /llms.txt e.g. https://svelte.dev/docs/svelte/what-are-runes/llms.txt).
4. How about the other documentation pages (manual pages)? If accepted for the docs should all pages be available in markdown format?
5. On a tangent: I noticed that threejs.org doesn't include a sitemap.xml and was wondering if this is something that should be added too (in a different PR)?

Motiviation:
I'm thinking about displaying some docs directly into the Needle Inspector extension or at least making it easier to query the relevant pages as markdown content when AI requests it.

This contribution is funded by Needle

[marwie]

The code here isn't sanitization - besides that It's very possible that. a lot of this can actually be removed (or removed altogether if another solution like plugins like jsdoc-to-markdown are preferred)

[Mugen87]

I've shared a script for producing llms.txt files in #31933. It's in this comment. Isn't that sufficient?

[marwie]

Sufficient for the markdown files per html?

Maybe - I've tried it with the plugin at first (similar to the script you shared) but the output also seems to need some postprocessing to remove HTML or use a handlebars template:

e.g. here **Default**: <code>LoopRepeat</code> many places use <code> instead of backticks

or:

<dl>
<dt><a href="#ArrayNode">ArrayNode</a> ⇐ <code>TempNode</code></dt>
<dd><p>ArrayNode represents a collection of nodes, typically created using the <a href="#array">array</a> function.</p>
<pre><code class="language-js">const colors = array( [
    vec3( 1, 0, 0 ),
    vec3( 0, 1, 0 ),
    vec3( 0, 0, 1 )
] );

const redColor = tintColors.element( 0 );
</code></pre>
</dd>
</dl>

So I tried if directly writing markdown from jsdoc would be easier (which turned into this PR). Maybe the HTML in there is also totally fine - I'm not 100% sure.

[Mugen87]

Closing in favor of #32673.