create a 'Client-side routing' docs page

[vlukashov]

When adding client-side route into a Vaadin app I want to read a docs page that tells me how to do that so that I do not have to look into the Flow source code or ask questions online when I am not sure how to do that.

DoD:

• A 'Client-side routing' docs page is added into the Client-centric development mode section of the ccdm branch of Flow docs (use the PR https://github.com/vaadin/flow-and-components-documentation/pull/729/files as an example)
• The page explains how to set up client-side routing in a Vaadin app:
  • prerequisites: Vaadin 15+, client-side bootstrapping (links to Client-side Bootstrapping)
  • How to show a client-side view (links to Vaadin Router)
  • How to show a server-side view (describes the flow.route API)
  • How to show a combination of client-side and server-side views inside the same parent layout
  • The effective order of navigation events: client and server (links to Flow Router navigation lifecycle and Vaadin Router lifecycle callbacks
  • How to add server-side navigation event handlers
  • How to add client-side navigation event handlers