Docs for reusable components

[JakobMiksch]

relates to #265 and #272

quote from @fschmenger in #265:

First of all I think we need an anchor for documenting reusable components / Wegue API - as all the documentation so far is mostly app-config related. My proposal would be to introduce something like reusable-components.md. We can then use this as the anchor for e.g. ModuleCard or MapOverlay documentation (see #269). Also all the Wegue templates (AppTemplate, AppFooter etc.) with their respective customization options and slots might go in there.

[fschmenger]

We could get this started partially for some components (ModuleCard, MapOverlay). For the AppTemplate related stuff I think it's worthwhile to wait until #276 is implemented.

[fschmenger]

I gave this a first draft within #269 to document the MapOvelay class. I tried to mimic the documentation structure of the vuetify UI components, which usually consist of several sections per component (e.g. Props, Events, Slots) and some separated examples.

For a first shot this is just a hardcoded documentation. Since the section will be all about components, it would be nice to come up with a Doxygen like solution, so the documentation lives in the code. There seem to be projects on the web that support vue / vuetify, but I didn't look into those so far. If you have any preferences, let me know.