Create docs about how to add secrets in OpenHands web #184
Description
We should document how to create and use secrets (specifically the OpenHands Web tab):
- How to set secrets in the UI.
- How to prompt the agent so that it can access secrets.
- That secrets are masked out for privacy reasons.
It may be necessary to clone and read the OpenHands/OpenHands and OpenHands/software-agent-sdk repos to understand how secrets work to create appropriate docs.