Write updated docs and port SEP-2663 content

[LucaButBoring]

Writes new overview docs and ports content from modelcontextprotocol/modelcontextprotocol#2663

[CaitieM20]

What about Progress notifications? can these also come on this stream? We could set the _meta field to include the taskId. This would preserve existing functionality and give folks a path to partial results?

Or we can omit for now and solve later. But we should add language to expressly disallow progress notifications on this stream then.

[LucaButBoring]

Highlighting this one for @kurtisvg to chime in on — I think for now we should explicitly disallow this?

We actually have three options here:

1. Allow progress notifications for tasks on subscriptions/listen, which is simple to implement but inconsistent with how progress works in all other cases. This is inconsistent with subscriptions/listen in that it imposes a disjoint requirement.
2. Disallow it, but continue to allow it piecemeal over SSE responses from individual tasks/get requests, which I've done and it's not easy, but this is also the status quo and is consistent with the subscriptions/listen spec.
3. Disallow it, and also disallow it over tasks/get: This is what I think we should do — it means progress effectively isn't usable with Tasks anymore, but leaves the door open for us to find a better home for progress information in Tasks later. This is inconsistent with subscriptions/listen in that it is more restrictive.

I'm going to disallow it explicitly period for now, but I'm laying out the options here so we can have informed discussions about it.

edit: disallowed in a0824a2

[LucaButBoring]

Addressed all feedback up to this point.