docs: add llms.txt support via starlight-llms-txt plugin - #709

Description of changes
Installed starlight-llms-txt and configured for Web Monetization. The plugin generates three routes at build time:

• /llms.txt - curated index with project description and section links
• /llms-full.txt - complete documentation in a single flat file
• /llms-small.txt - condensed version with asides and whitespace collapsed

Special configuration for Web Monetization:

• details block explains the two main audiences (developers/publishers integrating WM vs. supporters sending payments), clarifies terminology that differs from common usage (wallet address vs. the older "payment pointer", continuous payments, probabilistic revenue share), notes how WM relates to Open Payments and ILP, and calls out the current maintenance status of the WM tools (community-led, no longer proactively developed for new features) so agents don't assume active feature development
• customSets groups content into five named subsets (For content owners (publishers), For content consumers (supporters), Developers, Guides and tutorials, Resources) - this helps agents pull targeted content instead of the whole file
• optionalLinks points to the GitHub repo, the Web Monetization specification, and the Open Payments documentation that WM-enabled wallets use under the hood to execute payments

Also required a small fix in ExtensionLinks.astro: Astro.site is undefined when starlight-llms-txt renders page content through Astro's container API (rather than a normal page request), which crashed /llms-full.txt and /llms-small.txt on any page embedding this component. Added a guard so the optional UTM param is only set when Astro.site is available — no change to normal page rendering.