[AI Gateway] Simplify docs to use default gateway and improve changelog

[ethulia]

Summary

Screenshots (optional)

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 change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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/changelog/	@cloudflare/pm-changelogs, @cloudflare/pcx-technical-writing
/src/content/docs/ai-gateway/	@abhishekkankani, @palashgo, @thebongy, @roerohan, @kathayl, @mchenco, @zeke, @superhighfives, @bfirsh, @cloudflare/pcx-technical-writing

[mchenco]

/lilbonk review this PR

[github-actions]

Preview URL: https://7b8a8d39.preview.developers.cloudflare.com
Preview Branch URL: https://changelog-fixes.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/changelog/2026-03-02-default-gateway/	https://changelog-fixes.preview.developers.cloudflare.com/changelog/2026-03-02-default-gateway/
https://developers.cloudflare.com/ai-gateway/get-started/	https://changelog-fixes.preview.developers.cloudflare.com/ai-gateway/get-started/
https://developers.cloudflare.com/ai-gateway/features/unified-billing/	https://changelog-fixes.preview.developers.cloudflare.com/ai-gateway/features/unified-billing/
https://developers.cloudflare.com/ai-gateway/usage/chat-completion/	https://changelog-fixes.preview.developers.cloudflare.com/ai-gateway/usage/chat-completion/

[ask-bonk]

Review Summary

No critical issues found. 3 style consistency items to address.

Issues

MEDIUM - Inconsistent environment variable formatting:

• src/content/docs/ai-gateway/features/unified-billing.mdx:57-58 — Uses {account_id} and {CLOUDFLARE_TOKEN} instead of $CLOUDFLARE_ACCOUNT_ID and $CLOUDFLARE_API_TOKEN
• src/content/docs/ai-gateway/features/unified-billing.mdx:118-119 — Same issue in the ZDR example
• src/content/docs/ai-gateway/usage/chat-completion.mdx:22 — Uses {account_id} instead of $CLOUDFLARE_ACCOUNT_ID

These should use shell-style variable names to match the style used in get-started.mdx and the changelog.

Notes

The PR simplifies the getting started flow effectively by removing manual gateway creation steps and emphasizing the default gateway. All MDX syntax is correct, imports are present, and the changelog entry follows the proper format.

github run