Update Page.register_extension example in docs
Docs update - display full module path for extensions

Shorthands were deprecated in FeinCMS 1.6 and removed in FeinCMS 1.8
Merge pull request #517 from bmihelac/patch-9

Update Page.register_extension example in docs
Merge pull request #518 from bmihelac/patch-10

Docs update - display full module path for extensions