Docs: Interactivity API - Add documentation for getServerState() an…

…d `getServerContext()` (#66104)

* Add documentation on `getServerState()` and `getServerContext()`

* update heading level

* Undelete the "Conclusion"

* Fix link to the documentation

* update link to router docs

* clarify about context

* add reference to @wordpress/router

Co-authored-by: michalczaplinski <czapla@git.wordpress.org>
Co-authored-by: DAreRodz <darerodz@git.wordpress.org>