docs: Update N release notes for DP4

Archived the DP3 release notes, and set up the skeletal DP4 notes.
Also added information about changes to multi-window mode.

(Began this CL on mnc-io-docs, but CPing it to nyc-dev since that's
where we'll be doing the DP4 docs.)

See first comment for doc stage location.

bug: 28982873
bug: 28982952
Change-Id: I9dd8579db8b88788a89f046fe6cf2052390aade4