Refine page "URI mapping".
The current version has a focus on travel and multisite.
Where using travel for examples is fine - our docs for this topics should not only show the multisite aspect.

A hidden page from Ruth Multisite work (from Ruth) actually has "better" infos than the current one. (But for sure that one is again multisite related.)

What should the new version of the* Virtual URI mapping* ref. page provide?

• Explain the general principle of these mappings
• What mappings are installed by in our modules and bundles? Probably not all, but at least some of them should be showed and explained.
  • search on the config workspace with the term "virtualURIMapping" (https://goo.gl/Ha5Ffw)
• Provide a list of the possible arguments for the class attribute, info.magnolia.cms.beans.config.RegexpVirtualURIMapping is just one example. However, to keep it short, we do not have to explain them all.
• Give some hints and use cases how to use it
  • Remember the example from the "JSON page" ("Turning a template-based JSON provider into a RESTful API with virtualURIMapping", https://goo.gl/aYZIML)
• Mention the subapp Virtual URI mapping (on the about app), see screenshot

Keep in mind, that API currently undergoes some changes, see:

1. MAGNOLIA-7016 - Improve API of Virtual URI mappings
2. MAGNOLIA-3349 - Port Virtual URI mappings to a Registry

(2) is closed, but not yet integrated. It has to wait for (1).
mgeljic and oanh.thai know the exact details about when these changes.

This said, once the 2 dev tickets are done and integrated, we may have to change the "new" docs again.
However it is fine to start with it.