docs: add self-hosted GitLab OAuth setup guide

[mintlify]

Summary

Adds a new docs page at /deploy/gitlab-self-hosted walking customers through connecting a self-hosted GitLab instance to Mintlify via OAuth (the OAuth flow used by workflows — distinct from the existing deploy-token integration).

What it covers

• Prerequisites — admin access on the GitLab instance, public reachability from dashboard.mintlify.com, feature-flag enrollment.
• Set up the connection — registering the OAuth application on GitLab (with the right Redirect URI, scopes, Confidential/Trusted settings), copying credentials into Mintlify, authorizing, and selecting projects to enable webhooks on.
• Rotate credentials — when the GitLab-side secret rotates (which it does silently on edit), the delete-and-re-add flow.
• Troubleshooting — the four most common failure modes from real testing:
  • invalid_client after authorize (stale secret)
  • Webhook registration Invalid url given (GitLab outbound-request allowlist)
  • Missing consent screen (Trusted flag or pre-existing user grant)
  • Local dev URL mismatches (host.docker.internal)

Why now

This page is referenced from the dashboard's Settings → Git settings → GitLab OAuth → Self-hosted GitLab card via a "How to create the OAuth app" link. Companion to mintlify/server#5100 and mintlify/mint#7564, which add the underlying OAuth flow.

Heads-up

The link in the dashboard component currently points at https://mintlify.com/docs/settings/gitlab-self-hosted, but the docs site has no /settings/ namespace. The page lives at /deploy/gitlab-self-hosted (sibling to the existing GitLab guide). The dashboard PR needs a one-line update to match the new path.

Test plan

• Open the page in the docs preview and verify Steps render, Frame screenshots aren't broken (none are referenced — text-only initially), and internal anchor [Rotate credentials](#rotate-credentials) resolves.
• Spot-check that the new page appears in the left nav under the same group as /deploy/gitlab.
• Cross-check the URLs and admin paths against a real self-hosted GitLab instance once available.

Mintlify

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Note

Low Risk
Low risk: documentation-only changes plus a nav entry; no runtime code or behavior is modified.

Overview
Adds a new /deploy/gitlab-self-hosted doc that explains how to connect a self-hosted GitLab instance to Mintlify via OAuth for workflows (app registration, required scopes/redirect URI, authorization, and project webhook selection), with credential-rotation and troubleshooting guidance.

Updates docs.json navigation to include the new page under Deploy alongside the existing deploy/gitlab guide.

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[mintlify-development]

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Project	Status	Preview	Updated (UTC)
userauth	🟢 Ready	View Preview	May 7, 2026, 11:02 PM

[mintlify]

Preview deployment for your docs. Learn more about Mintlify Previews.

Project	Status	Preview	Updated (UTC)
mintlify	🟢 Ready	View Preview	May 7, 2026, 11:05 PM

[ethanpalm]

just some style edits but lgtm!