Option to store documentation images to a network location

As a user of QDoc, I need a capability in qdoc and generated html docs to optionally fetch images used in documentation from a network location, instead of storing everything within source (git repositories).

Justification:

• Using git to store images (or other binary files) doesn't make much sense, is inefficient
• Avoiding somewhat artificial size restrictions
• Easy updatability, images shouldn't need to pass CI

We need to gracefully handle the situation where network is not available (in offline docs).