add documentation for authentication via proxy

[yrobla]

Description

Added specific documentation for authenticating via proxy

Content

• New pages include a frontmatter section with title and description at a minimum
• Sidebar navigation (sidebars.ts) updated for added, deleted, reordered, or renamed files
• Redirects added to vercel.json for moved, renamed, or deleted pages (i.e., if the URL slug changed)

Reviews

• Content has been reviewed for technical accuracy
• Content has been reviewed for spelling, grammar, and style

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
docs-website	Ready	Preview	Comment	Oct 17, 2025 1:45pm

[danbarr]

This feels long enough to warrant its own guide - probably the authn/z how-to content needs to be a category with several pages covering run, proxy, and then the cedar policies as separate pages?

Another note, might be good to hold off on merging this into the auth-docs branch - the docs currently in that branch haven't been updated/fixed yet (they're outdated, probably wrong, and just a bit of a mess), so I wouldn't want to merge good into bad just yet? @jhrozek and @ChrisJBurns have started tag-teaming those I think.

Overall I'll plan to hold my own reviews until the auth docs are technically vetted/ready in the auth-docs branch.