docs: document streaming EXPLAIN ANALYZE HTTP API

[discord9]

What changed

Document the POST /v1/sql/analyze/stream (SSE) endpoint contract added in v1.2 (upstream GreptimeDB #8380): request parameters, EXPLAIN ANALYZE VERBOSE statement restrictions, the metrics/final/canceled/error SSE events and payload fields, snapshot semantics (complete best-effort snapshot with adaptive coalescing), client-disconnect cancellation, and the no resume/reconnect lifecycle.

Closes #2719.

Scope

• Documentation versions: Nightly, 1.2
• Languages: English, Chinese

Verification

• Contract verified against GreptimeDB v1.2 source (src/servers/src/http/handler.rs SSE event generation; endpoint registration in src/servers/src/http.rs; config default http.experimental_enable_explain_analyze_stream = true, snapshot_interval_ms default 5000 clamped to [1000, 60000]).
• EN/ZH and Nightly/v1.2 copies are line-aligned.
• markdownlint config in repo disables all rules; manual structural checks passed. Docusaurus build not run (no node runtime available).

Checklist

• I verified the content against the applicable GreptimeDB version.
• I updated the relevant documentation versions and languages, or explained why not.
• I checked changed links and anchors.
• I updated navigation when the document structure changed.

[fengjiachun]

The example uses curl -F, which sends multipart/form-data, but this handler extracts Axum Form<SqlQuery>, which accepts application/x-www-form-urlencoded. Because the unsupported media type is ignored and no query-string sql is supplied, this command reaches sql parameter is required instead of opening an SSE stream. Please use --data-urlencode 'sql=EXPLAIN ANALYZE VERBOSE SELECT * FROM monitor' and update all four copies so the documented example works.