[Feature][Dashboard] Add GitLab as a valid data source for Contributor Experience Dashboard

[bujjibabukatta]

Summary

Closes #8874

This PR makes GitLab a valid data source for the Contributor Experience
Dashboard, which was previously only wired up for GitHub.

Root Cause

The DEVLAKE_DASHBOARD_URL_GITLAB environment variable existed in the
type definitions and DashboardURLMap but was pointing to the general
GitLab dashboard (msSjEq97z) instead of the Contributor Experience
dashboard (bwsP5Nz4z). The Contributor Experience dashboard itself uses
only normalized DevLake domain-layer tables (issues, pull_requests,
repos, board_issues) with no GitHub-specific SQL, so it works for
GitLab data out of the box.

Changes

File	Change
config-ui/env.example	Point DEVLAKE_DASHBOARD_URL_GITLAB (and GITHUB) to the Contributor Experience dashboard
grafana/dashboards/mysql/contributor-experience.json	Add GitLab tag; update intro panel to list both GitHub and GitLab as supported sources
grafana/dashboards/postgresql/contributor-experience.json	Same as above for PostgreSQL variant
config-ui/public/onboard/step-3/gitlab.md	Add Q5 to inform GitLab users which dashboard appears after collection

How It Works

No backend changes are required. DevLake's GitLab plugin normalizes
merge requests into the pull_requests table and issues into the
issues table — the same tables the Contributor Experience dashboard
queries. GitLab users completing the onboarding flow will now land on
the Contributor Experience dashboard (showing Issue Response Time, PR
Review Time, Good First Issues, etc.) instead of the general GitLab
dashboard.

Testing

1. Set up a GitLab connection via the onboarding flow
2. Complete data collection
3. Verify the "Check Dashboard" button opens the Contributor Experience
   dashboard and metrics populate correctly from GitLab data