docs(confluence): add Confluence knowledge integration guide

[christestet]

Important

This PR is a DRAFT — it depends on an unreleased feature in the main repo. Please review together with the feature PR once merged.

Related feature PR: open-webui/open-webui#21727

Summary

• Adds documentation for the new Confluence knowledge integration
• Targets dev branch as the feature is not yet released
• File: docs/tutorials/integrations/confluence.mdx

What's in this doc

1. Admin setup — enable the integration, set Confluence base URL and deployment type; env var alternatives (ENABLE_CONFLUENCE_INTEGRATION, CONFLUENCE_BASE_URL, CONFLUENCE_DEPLOYMENT_TYPE)
2. Per-user credentials — Cloud (email + API token from id.atlassian.com) vs. Data Center (Personal Access Token or username/password), stored per-user (no shared service account)
3. Import workflow — step-by-step: select space → browse hierarchical page tree → select pages/subtrees → import with progress feedback
4. Architecture reference — table of backend components, API endpoints, config variables, auth methods
5. Troubleshooting — missing menu option, failed connection test, empty spaces list, failed imports, stale content

Test plan

• Doc renders correctly in Docusaurus (mdx, tables, admonitions)
• Feature PR merged → convert to ready for review
• Verify sidebar position (33) doesn't conflict with adjacent entries

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[pr-validator-bot]

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