[enhancement] Add llms.txt support

[`llms.txt`](https://llmstxt.org/) is a hopeful new standard for telling LLMs how to understand the best parts of your website. Apparently Cursor and Claude already support it as a way to load context. This could be a shortcut for getting good-quality code samples into all those AI vibe coding copilots out there.

AC:

1. An `llms.txt` file is there in the root of the web server.
1. This file gives minimal, concise context on what Holochain's about and how you code a hApp.
2. It also links out to important internal URLs necessary for generating good hApp code.
3. It also links to our generated Rustdoc and JSDoc.

Optional:

1. An `Optional` section in the `llms.txt` file that links to selected supporting third-party info.
2. The internal URLs listed in the file (from AC 1.2) are served as Markdown, not HTML.

Start by locating the static web-server root and the existing documentation URL structure in this hosted documentation repository. Add the root llms.txt with concise Holochain and hApp-coding context, links to key internal pages, and links to generated Rustdoc and JSDoc; verify that the listed URLs resolve and that any optional Markdown serving is addressed.

Written by the indexing model from the issue text.