[w3c/manifest] Simple Example and Complex Example (#553)

I've noticed some manifest members are not included in our current example.

I'm thinking we should have two examples:
* A simple example with a small set of the most common fields (ex: name, icons, etc.)
* A complex example with a complete (or near complete) set of all fields (ex: prefer_related_applications, etc.)

If folks agree, please comment and I'm glad to draft a PR.  /cc @marcoscaceres @kenchris @mounirlamouri @anssiko 

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