docs(ai-sre): GitLab App single OAuth path for every instance

[ysyneu]

Summary

• Drop the one-click "Connect GitLab.com" vendor OAuth app path from the AI-SRE Apps doc.
• Promote the existing OAuth-app registration walkthrough to the single connection path for every GitLab instance — GitLab.com, JihuLab (jihulab.com SaaS and private distributions), and self-managed instances all register an OAuth app the same way.
• Describe the wizard as one flow: enter instance URL (gitlab.com prefilled) → register the OAuth app with the shown Redirect URI and paste Application ID/Secret → authorize in the popup → pick groups/projects.
• Note that on gitlab.com, a user-owned application (User Settings → Applications) also works if you're not a group Owner.
• zh edited first, en mirrors it. All other content (bot provisioning, Developer-level permissions, disconnect, paid-namespace warning) is unchanged.

Test plan

• Verified no other docs pages reference a GitLab one-click connect path (grep -rn "一键\|one-click" --include="*.mdx" across the repo, filtered for gitlab — no hits)
• Checked MDX renders (no bare {} introduced, headings/anchors preserved)