Docs: Improve Custom Sandbox Documentation

[h55nick]

What problem or use case are you trying to solve? I tried to setup a custom sandbox and while it wasn't complicated, the current documentation wasn't enough to do it seamlessly.

Current documentation: https://docs.all-hands.dev/modules/usage/how-to/custom-sandbox-guide Code: https://github.com/All-Hands-AI/OpenHands/blob/6184b9d7f427a3e33285ea1d15262912368eaa0e/docs/modules/usage/how-to/custom-sandbox-guide.md?plain=1#L49

Describe the UX of the solution you'd like Improve documentation with a more detailed approach

Do you have thoughts on the technical implementation? N/A

Describe alternatives you've considered N/A

Additional context Slack thread: https://openhands-ai.slack.com/archives/C06U8UTKSAD/p1732566191176979