cubicweb #2005161 improve docstrings of [resolved]

I think we could improve the current documentation about how core cubicweb is configured. This could be achieve with dedicated section for:

Mode:Various mode.
Resource:What kind of resource cubicweb is looking for and where.
Virtual Env:Explain how is handle this first (not so) special case.
Development Mode:Explain how is handle this second special case.
done in3.13.10
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