Document the streaming EXPLAIN ANALYZE HTTP API

[fengjiachun]

Context

GreptimeDB v1.2 introduces a streaming SQL analysis endpoint:

• Upstream: feat: stream explain analyze metrics over http greptimedb#8380
• Endpoint: POST /v1/sql/analyze/stream
• Transport: Server-Sent Events (SSE)

The current docs only expose the related configuration and do not document the API contract.

Documentation work

Add HTTP API/reference documentation covering:

• A request example using one EXPLAIN ANALYZE VERBOSE statement.
• Accepted format constraints and rejection of non-explain or multi-statement requests.
• SSE event types: metrics, final, canceled, and error.
• Metrics as complete, best-effort snapshots rather than deltas, including snapshot coalescing.
• Terminal event behavior and client disconnect cancellation.
• No resume, reconnect, or detached-query lifecycle.
• The v1.2 default for the related configuration, verified against the v1.2 source/tag.

Scope

Update both English and Chinese documentation in:

• Nightly
• v1.2

Completion criteria

• The endpoint has a complete request and event-stream example.
• Lifecycle, cancellation, and snapshot semantics are explicit.
• Version-sensitive defaults are verified against v1.2.
• English and Chinese content is aligned.
• Affected locale builds pass.