docs: add user tokens and granular scopes page

[hugocasa]

Summary

• New dedicated documentation page for user tokens (core_concepts/user_tokens)
• Covers token creation, granular scopes (scope format, domains, path patterns), webhook-specific tokens, and usage
• Includes screenshots of the token creation form and scope selector UI
• Adds backlinks from authentication, webhooks, HTTP triggers, and core concepts index pages
• Updates HTTP triggers docs to mention pre-scoped token generation from trigger config

Test plan

• Verify the new page renders correctly at /docs/core_concepts/user_tokens
• Check all internal links resolve (sidebar, backlinks from auth/webhooks/http_triggers pages)
• Verify screenshots display properly

🤖 Generated with Claude Code

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Branch Preview URL:	https://docs-user-tokens-granular-sc.windmilldocs.pages.dev

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