Add docs for Sandbox SDK

[ghostwriternr]

This PR is WIP. I'll update the summary and details when ready for review.

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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.

[whoiskatrin]

can you please pricing page as discussed?

[whoiskatrin]

Sandbox SDK llms-full.txt
Developer Platform llms-full.txt

return nothing, 404, is that expected?

[threepointone]

the build failure is legit

[threepointone]

this is really fkin good. let's pass ci and land it, we can always tweak later

[mikenomitch]

I might combine the middle two boxes into a single one. - I guess this is saying app calls stub, stub calls DO? But looks like its making an extra hop here