add docs for keepAlive flag

[whoiskatrin]

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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/docs/sandbox/	@whoiskatrin, @ghostwriternr, @cloudflare/pcx-technical-writing, @cloudflare/ai-agents

[ghostwriternr]

Nice! Some minor comments. And we should also update the API reference since there is now setKeepAlive too.

[github-actions]

Preview URL: https://db7f857f.preview.developers.cloudflare.com
Preview Branch URL: https://add-keep-alive-docs-sandbox.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/sandbox/configuration/sandbox-options/	https://add-keep-alive-docs-sandbox.preview.developers.cloudflare.com/sandbox/configuration/sandbox-options/
https://developers.cloudflare.com/sandbox/guides/background-processes/	https://add-keep-alive-docs-sandbox.preview.developers.cloudflare.com/sandbox/guides/background-processes/
https://developers.cloudflare.com/sandbox/concepts/sandboxes/	https://add-keep-alive-docs-sandbox.preview.developers.cloudflare.com/sandbox/concepts/sandboxes/
https://developers.cloudflare.com/sandbox/api/sessions/	https://add-keep-alive-docs-sandbox.preview.developers.cloudflare.com/sandbox/api/sessions/
https://developers.cloudflare.com/sandbox/configuration/	https://add-keep-alive-docs-sandbox.preview.developers.cloudflare.com/sandbox/configuration/