docs: sync dewey Vertex AI `region: global` documentation

[yvonnedevlinrh]

What changed

PR #88 (unbound-force/dewey#88) added support for region: global in Vertex AI provider configuration. When region is set to global, requests are routed to aiplatform.googleapis.com (nearest available region) instead of a region-prefixed hostname.

What needs updating

• Vertex AI setup/configuration page — document global as a valid region value alongside regional examples like us-east5, us-central1
• Provider configuration reference — note that global routes to the nearest available region and uses a different hostname (aiplatform.googleapis.com vs {region}-aiplatform.googleapis.com)

Affected website pages

Pages under content/docs/ covering Dewey's provider configuration or Vertex AI setup.

Repo

unbound-force/website