Document SDK-owned browser conversation event streams

[neubig]

Status: ready-for-dev

Document the browser ConversationEventStream client added for OpenHands/software-agent-sdk#5012: endpoint construction, first-frame authentication, replay, lifecycle callbacks, reconnect/cleanup, and disconnected-send handling. Include an unreleased API note and publish alongside the SDK release. Acceptance: usable example, navigation link, and successful documentation link checks.