chore: upgrade to rspress 2 & add llms.txt

[jbroma]

Summary

• - upgraded rspress to 2.0.0-beta.3
• - added @rspress/plugin-llms
• - wrap Announcement component in NoSSR
• - fixed llms.txt generation for scenario without locale
• - added sitemap plugin for SEO

Test plan

n/a

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Latest commit: f668edd

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