v8 is missing the majority of v7 topics

[ghost]

Make sure to check the existing issues in this repository

Please, tell us what's the problem?

[ ] A typo
[ ] Wrong documentation
[ ] Improvement of existing article
[x] Missing documentation
[ ] New article needed

Please, tell us more details

It looks like the v8 docs are really thin compared to v7 docs. For example, nothing about themes and defining a skin, core concepts, core modules, etc...

Perhaps there needs to be a TODO issue that shows what's missing and needs to be brought over.