Add build notifications template to event subscriptions docs

[yomna-shousha]

Summary

• Adds a "Send build notifications" section to the Workers Builds event subscriptions page
• Includes Deploy to Cloudflare button for the builds notifications template
• Shows example Slack notification screenshot
• Mentions template works with Slack, Discord, or any webhook endpoint

Related to changelog PR #27067

[github-actions]

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

Pattern	Owners
/src/content/docs/workers/ci-cd/	@irvinebroque, @aninibread, @GregBrimble, @cloudflare/pcx-technical-writing, @yomna-shousha

[github-actions]

Preview URL: https://aa103c3f.preview.developers.cloudflare.com
Preview Branch URL: https://yomna-subscriptions-builds-docs.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/workers/ci-cd/builds/event-subscriptions/	https://yomna-subscriptions-builds-docs.preview.developers.cloudflare.com/workers/ci-cd/builds/event-subscriptions/