docs: add self-hosted setup page to navigation and link from overview

[thesandlord]

Summary

Makes the self-hosted setup documentation page visible in the navigation and adds a link to it from the overview page. This follows up on PR #2930 which added the setup content but left the page hidden.

Changes:

• Removed hidden: true from the "Set up self-hosted documentation" page in docs.yml
• Added a link to the setup page from the "Setup process" section in the self-hosted overview page

Review & Testing Checklist for Human

• Verify the setup page appears in the Self-hosted section of the navigation sidebar
• Verify the link in the overview page works and navigates to the setup page

Recommended test plan: Use the preview deployment to navigate to the Self-hosted section and confirm both the navigation entry and the inline link work correctly.

Notes

• Tested locally: Navigation sidebar shows the setup page, and the inline link navigates correctly
• Requested by: Sandeep Dinesh (@thesandlord)
• Devin session: https://app.devin.ai/sessions/9b543131c5a44066a8d51a418819d524

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