Guide: router module (#1279)

* Router module initial commit

* Add the router.md itself

* Enqueue only on enact_candidate

* Initialization concerns

* Ignore the snippet

* Update roadmap/implementors-guide/src/runtime/router.md

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

* Dispatch messages on finalization

* More fixes from review

* Update roadmap/implementors-guide/src/SUMMARY.md

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

* Adds router to initializer order

* Adds messages.md

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>