Update llms.txt docs for partial auth support

[mintlify]

Summary

• Updated llms.txt and authentication-setup pages to reflect that partially authenticated sites now serve llms.txt and llms-full.txt publicly with only public pages included
• Updated feature availability table to distinguish between fully and partially authenticated behavior
• Applied changes across all locales (en, zh, fr, es)

Triggered by mintlify/server#4727.

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Note

Low Risk
Low risk documentation-only change that updates guidance around authentication visibility for llms.txt/llms-full.txt across locales; no runtime behavior is modified.

Overview
Updates the llms.txt documentation to distinguish fully authenticated sites (files require auth) from partially authenticated sites (files are publicly accessible and include only public pages), while still noting that pages in user groups are excluded.

Aligns the Authentication docs’ Feature availability section and table to reflect this new partially-authenticated behavior for llms.txt/llms-full.txt, and applies the same wording updates across EN/ES/FR/ZH locales.

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[ethanpalm]

Closing: no evidence this feature has shipped to production. This PR references mintlify/server#4727 for partial auth llms.txt support, but that PR has not merged to server main (current tip is #4709). Ryan's standup today noted merging this as a to-do item, not a done item. If this feature ships, a new docs PR can be opened at that time.