[import] Document links update and provide helper function

[kptdobe]

Links need to be updated (make absolute on the .page instance) - pb, during import we do not know the target .page host name thus the default import cannot do it and it is responsibility of the dev to do it. We should update the document to describe the todos and provide a makeLinksAbsolute helper method.

[kptdobe]

The AEM cli sets a hlx:proxyUrl meta property. We might be able to use it and automatically update the links... needs investigations.

[kptdobe]

Quickly looked into it, the meta property approach is not a good option: the property would have to be injected into the static html files of the Importer UI (which is ugly) and would also imply that the import.js knows that in its "execution context" there is this meta property. Then if you want to use the import.js in a different context, like a Node.JS importer, you need to make sure it provides this meta. Not nice.