Clarify the theme "Layers" UI

[tordans]

A couple of things are unclear on this page…

• [x] What are subcategories? I don't thing that term is used in studio elsewhere. Is there a different term that can be used?
• [x] I guess the fact that nothing is selected at the moment is due to https://github.com/pietervdvn/MapComplete/issues/1922 ?
• [ ] But why is this panel active wen I open the Theme Edit UI … it triggers me to do something, but I am pretty sure I did all I need to do already
• [ ] Why is the "berlin_emergency_water_pump_layer_v2 (made by you)" option displayed twice? I removed all layers except one from the Layer Edit UI, so it should be present only once.
• [x] Please update the big info text at the top to be specific for the Studio. Most of this is not relevant for studio and AFAIK half of it is strongly discouraged by console infos (referring to the "layer as part of theme), so there is not need to explain it prominently here (and it's not even possible to do with studio).
• [x] It would also be nice to render the info text as markdown so it does not look so broken with all the ```

[pietervdvn]

It would also be nice to render the info text as markdown so it does not look so broken with all the ```

The markdown-parser I used didn't support this notation. Fixed now