Organizing documentation

This issue was triggered by the insightful comments by @filmaj in chaoss/grimoirelab-sirmordred#167

To summarize, now we have the following documentation:

* Some README.md files in source code repos, including some in subdirectories of them.
* Some automatically built documentation from source code comments. In some cases (well, in one case) it is available in readthedocs: [Perceval documentation](http://perceval.readthedocs.io/)
* The [Grimoirelab Tutorial](https://chaoss.github.io/grimoirelab-tutorial/), produced automatically from md files in docs dir in this repo.
* Some interesting information here and there in some issues.

I agree with @filmaj that we should better organize it. In principle, the idea (I think) was:

* README files for stuff like how to configure and run, how to contribute (maybe in separate CONTRIBUTING files), how to run tests and do other development-related stuff, and links to further documentation. In short, this would be information mainly for developers, and for letting users know the very basics, and where to find more. I would say this would be a kind of a reference manual for users, and in part (except for everything in the next item) for developers.
* Automatically generated documentation from comments in the source code. This should be for the rest of developer-related information, including definition of the APIs, for example.
* Tutorial. This was intended not as a reference manual, but as a tutorial of how to get stuff done. This should be the second thing a user would consult, after the README files that would explain the very basics, and after the detailed user guides that would be also a part of those README.

What do you (anyone reading this issue) think about this organization? Is still a sensible schema? (I'm not saying it is implemented this way now, just checking whether you find it convenient to set it as a goal).