docs: JSON-RPC response streaming for traces and logs

[svlachakis]

What

Documents JSON-RPC response streaming, which shipped in 1.39.0 and had no prose anywhere: a new Streaming responses section on the JSON-RPC server page, with a subsection each for traces and logs.

The two configuration options already appear in the generated configuration reference, but nothing told an operator what streaming does, that traces stream by default, or why the logs one is opt-in. Operators running 1.39 have the trace behaviour today without knowing it.

Content

• What streaming is for: a response far larger than the request that asked for it is written to the transport as it is produced, so the memory a request costs is bounded by the stream rather than by the size of its result, and the first bytes arrive before the work finishes.
• Traces: JsonRpc.EnableTracingStreamMode, on by default, with the per-call streamMode override on debug_trace* and a Cast example. States plainly that streaming bounds memory and lowers time to first byte but does not make a trace faster, since tracing re-executes the block.
• Logs: JsonRpc.EnableLogsStreamMode, off by default, and the reason it is opt-in: a streamed response stops at the per-response limits instead of buffering the whole result and returning a limit error, and authenticated requests are not subject to those limits.

Style

Behaviour only, no numbers; every option links to its entry in the generated configuration reference. All four anchors this section links to already exist there, so it adds no broken anchor (the build's remaining warnings are the pre-existing FlatDb.History* and ArchiveProof* ones, which appear when the reference is regenerated for 2.0).