Sandbox docs

[josephjclark]

First pass of basic docs for sandboxes

Screenshots deliberately excluded. I might come back and add a few in later

AI Usage

Please disclose how you've used AI in this work (it's cool, we just want to
know!):

• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

You can read more details in our
Responsible AI Policy