Add llms.txt for AI/LLM discoverability

[abyworkings-coder]

What

Adds an llms.txt file at the site root (served from .vuepress/public/llms.txt, following the same static-passthrough convention as opensearch.xml and icon.png), giving AI tools and LLM-based assistants a curated, low-noise index of the documentation.

Why

llms.txt is an emerging convention (like robots.txt or sitemap.xml) for giving LLM-based tools a curated entry point into a site's docs, rather than requiring them to crawl or guess through the full site. Given nushell.github.io has 700+ individual command pages and 450+ blog posts, an exhaustive dump wouldn't be useful — this file instead curates the highest-value ~80 pages: getting-started guides, core language concepts, scripting/extensibility docs, configuration, the commands index and category pages, the cookbook, and the language guide.

What's included

• Getting Started (installation, quick tour, fundamentals)
• Migration guides (coming from bash/PowerShell/CMD)
• Core language concepts (types, variables, control flow, tables, records, lists, strings)
• Scripting & extensibility (hooks, aliases, completions, plugins, dataframes, testing)
• Configuration & environment
• Commands reference (index + key categories, not all 700+ individual pages)
• Cookbook and language guide indexes
• Optional: contributor book, contributing guide

Verified every linked URL returns 200 on the live site before opening this PR.

Test plan

• Confirmed no existing llms.txt (404 before this change)
• Confirmed .vuepress/public/ is the static-passthrough directory used for other root-level static files (opensearch.xml, icon.png), both live at 200
• Spot-checked all major linked paths (book, commands, cookbook, lang-guide, blog indexes, category pages) return 200 live

[fdncred]

I generally don't have a problem with this but I look at it and think it's just one more thing to keep updated when the site changes. Hopefully someone can keep up with that.