Reorganize the docs with a strong focus on the user

**Is your feature request related to a problem? Please describe.**
Related to https://github.com/netlify/netlify-cms/issues/598

Short version: Currently the docs are written the way the system works. I'd like to explore the option of reorganize it according to how a user might think.

**Describe the solution you'd like**
I will provide ideas and examples over time to come up with one. I would love to hear thoughts and feedback.

**Developers vs editors vs contributors**
In light of https://github.com/netlify/netlify-cms/issues/1340 and https://github.com/netlify/netlify-cms/issues/1038:

The docs (and eventually the entire website) will need to accommodate developers, editors and contributors alike. We should split content based on your profile. Upon entering the docs homepage you should have the option to choose between those three options before getting started. You should be able to easily switch between guides since you can have multiple roles at the same time.

**Considerations**
- Different areas of the CMS should have their own top level section. (Collections, widgets)
- DRY content. Currently the same topics can be found on different places in the docs.
- Sort the top level sections by things you need to do first, secondly etc.
- Explore flows based on user needs (I need my site to be stored on github, hosted without netlify identity, users should sign in with their email etc)
- Design. The docs are quite different from the Netlify visual identity and the look and feel of the CMS itself. Don't mind too much, but we could try to bridge some gaps.
- Navigation depth. Pages should be broken up in a way that you don't need to navigate by clicking anchor links IMHO.

**TL;DR** - First ideas put into practice: