+str #15091 Add time driven operators

* TimerTransformer with support for named timers
* dropWithin
* takeWithin
* groupedWithin
=pro silence github tagging (curl)
Merge pull request #15311 from ktoso/pro-silence-curl-2-3-dev

=pro silence github tagging curl (pr validation)
Merge pull request #15245 from akka/wip-15091-stream-time-driven-patriknw

+str #15091 Add time driven operators
+str #15218 Add mapFuture operator
=pro in akka-docs(-dev) watch *.rst sources
Merge pull request #15325 from spray/w/add-docs-watches

add watchSources for `*.rst` files
Merge pull request #15281 from akka/wip-15218-mapFuture-patriknw

+str #15218 Add mapFuture operator
=str #15353 propagate error if TimerTransformer.onTimer throws exception
+str #15288 Add user support for Producer backed by an actor
Merge pull request #15297 from akka/wip-15288-actor-producer-patriknw

+str #15288 Add user support for Producer backed by an actor
=str #15355: fix race with ExposedPublisher messaage in tee

 - this change makes the secondary subscription lazy
=pro #15070 link to release-2.3-dev from ScalaDoc
Merge pull request #15372 from akka/wip-15070-scaladoc-link-∂π

=pro #15070 link to release-2.3-dev from ScalaDoc
scalariform ActorProducer
Merge pull request #15354 from spray/w/fix-15353

=str propagate error if TimerTransformer.onTimer throws exception, fixes #15353
Fix Duct#append type parameter
=str #15347 in `concat` propagate errors in secondary upstream properly
Merge pull request #15386 from spray/w/fix-15347

=str #15347 in `concat` propagate errors in secondary upstream properly
=str Improve `merge` signature