[AI Gateway]: Add documentation for service account based auth and unified API on Vertex

[thebongy]

Summary

• Added documentation for using Google Vertex AI through AI Gateway's unified chat completions API with OpenAI SDK and cURL examples
• Added authentication section on Vertex covering two methods: service account JSON (with automatic token generation and caching) and direct access tokens
• Reorganized existing provider-specific endpoint examples and added links to relevant AI Gateway documentation

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/ai-gateway/	@abhishekkankani, @palashgo, @thebongy, @kathayl, @mchenco, @kodster28, @cloudflare/pcx-technical-writing

[github-actions]

Preview URL: https://6e8466cb.preview.developers.cloudflare.com
Preview Branch URL: https://rbansal-ai-gateway-vertex-docs.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/ai-gateway/usage/providers/vertex/	https://rbansal-ai-gateway-vertex-docs.preview.developers.cloudflare.com/ai-gateway/usage/providers/vertex/

[cloudflare-workers-and-pages]

Deploying with    Cloudflare Workers

The latest updates on your project. Learn more about integrating Git with Workers.

Status	Name	Latest Commit	Updated (UTC)
❌ Deployment failed View logs	client-prod	fa18395	Oct 01 2025, 05:48 PM

[sendmailtouzzal-spec]

Error code 522