feat: add GitLab OAuth provider docs page

[Copilot]

fix: #944

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[hsluoyz]

@copilot also need to update the sidebar

[Copilot]

@copilot also need to update the sidebar

Updated the sidebar to include GitLab in the OAuth providers list (commit f27fa94).

GitLab now appears in the sidebar navigation and on the OAuth category page:

The GitLab documentation page is also accessible with all required information: