Add LLM-friendly documentation export feature

[breitburg]

Implements automatic generation of LLM-friendly text files during Jekyll build:

• /llms/guides.txt: All 96 developer guides
• /llms/api-docs.txt: C, Android, iOS, JavaScript, and Rocky.js API documentation
• /llms/community.txt: Community tools, apps, and libraries

Features:

• Clean markdown format with minimal metadata (title + URL)
• Automatic Liquid tag/variable removal
• Filters out redirect pages and stub content
• Organized with "---" separators between pages
• Only generates files with actual content

Architecture:

• lib/llms_txt_builder.rb: Reusable builder module (DRY principle)
• plugins/generator_llms_txt.rb: Jekyll generator plugin (runs at :lowest priority)

[breitburg]

Should work, but api-docs.txt will have limited content in local development since the DOCS_URL environment variable isn't set and the SDK documentation ZIP files aren't available locally.