[WIP] add custom property example, fix location to en

I have no clue how to embed that contents into the page.yml in a way to be able to add multilang seoMetadata and multilang title/body. 
So the SeoContent is available in one language atm. I just fixed the lang to 'en', cause without that strange stuff happens in backend with {locale} in the route. So the SeoContent should appear in one locale only: en.

I also added an example for a extra properties. But i have just seen that my admin form isn't that what i expect it to be: add and delete properties in a collection of entities.