Hi @Edun,
Welcome to the community! 
and thank you for trying out SuiteCRM 8 and your feedback.
Regarding the questions you’ve mentioned:
1. Full documentation
Our plan is to continue releasing more and more documentation as we go along.
We are focusing on the new parts first. Mostly the new front-end extension framework and how to extend the new Suite 8 backend.
We have just released the first articles on the front end extensions, you can find the on Developer Guide. From this guide the current status is:
- Front end Architecture intro - Released and to be improved with more information later.
- Front-End Extension Setup - Released
- Adding a Custom Sidebar Widget - Released. (Note: also serves as base example for extending components)
- Adding a Custom Chart - In review, should be out soon
- Adding Custom Field Logic
- Adding a Custom Field - being worked on
- Adding a Custom Validator - being worked on
- More to come
2. Documentation Repo
Yes, its the following one.
3. Module loader and custom modules
From a metadata perspective, you should be able to add all new module metadata using the existing module structure.
The module loader has been updated to support adding SuiteCRM 8 side extensions to the /extensions folder. More documentation on that soon.
Regarding the module architecture, at the moment is mostly the same, with same metadata and same place to add the files. However, there are some new module specific files that can only be added on Suite 8 side, like “Statistics” (chart data and others) for example.
Hope this helps. If you have more questions or any of the above wasn’t clear, please feel free to reach out.