Update explainer to cover recent features

[jan-ivar]

Our explainer.md hasn't been updated since January 2023. It needs to be updated to include features since then. It also lacks pipeTo / pipeThrough in examples.

See also #693 which splits out the work to cover protocols.

[wilaw]

Also link in the new sub-protocol explainer https://github.com/w3c/webtransport/blob/main/explainers/subprotocol_negotiation.md

[wilaw]

@jan-ivar to update examples. @wilaw to update prose. Note the the needs-test label is a handy pointer to what's new.

[wilaw]

Add new example to explainer and spec for setting sendGroup and sendOrder, perhaps across multiple streams and datagramwritables.