Move tutorials

[TobiKattmann]

Hi all,

this PR is a followup to #37
As already mentioned in #37 I would like to shadow the /su2code/Tutorials folder for the tutorial section of the website. This provides a bit easier to understand structure and I think it is beneficial if one can simply copy the folders together to have everything "merged" to work on -> that requires _tutorial, tutorial_files (new) and /su2code/Tutorials to have the same folder structure... Much like Testcases in the code repo.
The code root has also ~15 less folders with this.

What I did: Created a tutorials_files folder following the _<name> + <name>_files convention which is kinda already there. Using there the same folder structure as in '_tutorial' but adding an 'images' folder to each "leaf". Copying the tutorial images over into that structure which makes all the specific tutorial folders in the code root unnecessary. Adapting the relative paths to the images in the *md files.
I tested each tutorial website locally and jekyll is so nice to complain in the terminal if it cannot find an image. So as far as I can tell nothing changes in the website that is seen... it is just under the hood that changes a bit.

Unfortunately it is not really straightforward possible to keep the markdown tutorial files and the images close togehter in on structure. I am not sure why that is but I am not alone with that idea. That's why there are the name_files directories for images and _name for the content that is processed.

Let me know what you think, Tobi

[TobiKattmann]

Merging this in now 🦖 If there are any issues let me know