Include a section on SVGs

[saberraz]

We do not ship the app with the default SVGs in QGIS or indeed any other external SVGs which user may have used for styling. It would be good to add a paragraph inside Project Preparation and inform users how to "Embed" svgs within the project, so it will appear correctly in the app.

[PeterPetrik]

MerginMaps/qgis-plugin#209

[saberraz]

@alex-cit the current doc refers to packaging the SVGs (https://merginmaps.com/docs/gis/features/#layer-symbology), it is a better choice to embed the SVGs:

[PeterPetrik]

There should be now new validation in plugin - so users should be notified if they do not embed it