[WIP] Add documentation

[suargi]

Add documentation from my personal notes, pull requests and the template config file into SU2 website.

I have been taking notes on how to use SU2 and important features that I think are missing in the documentation. I would like to contribute to the documentation. Besides, in my opinion the template config file and pull requests are not a source for documentation, although the config file can support the documentation. So, I gathered major point from them and included into the website documentation.

It is a little bit difficult to link the website content and the two references in https://github.com/su2code/Documentation/tree/master/Articles as they have "different" content and the papers are somehow outdated.

There are certain aspects that I am not sure and would be nice if you could double check. I have marked them with a "?" for further discussion.

Sorry for grammar errors :)

[pcarruscag]

Awesome! Thank you for adding these much-needed pages on gradients/limiters and turbulence models.
I have still have to finish reading the turbulence part, but here are some suggestions:

[pcarruscag]

Suggested change

	Thin Shear Layer gradient reconstruction is always used for the construction of the Jacobian.

That would be "fluxes" theory.

[suargi]

You are right in the sense that the Jacobian construction belongs to a "fluxes" section. However the discussion on which gradient computation method is used for its construction might belong to the "Gradients" section.

[pcarruscag]

I like this idea, but it would cause a severe break of backwards compatibility, we will have to stick with KIND_TURB_MODEL and specify new corrections separately.

[suargi]

Alright. We can discuss this in su2code/SU2#1066.

[suargi]

This PR is quite old and I do not remember exactly what is its status. I will create additional but smaller pull request, e.g., #81 in order to include all the updated document ion from this PR. At the end, this PR will be useless and can be closed.