maintaining docs for site structure

[choldgraf]

This adds some simple documentation explaining the high-level site structure.

[choldgraf]

ok I'm just gonna merge these :-) (just a note that I am at least personally fine with any of y'all just merging stuff right away)

[BENR0]

Hey @choldgraf I think there is just a small mistake in the last sentence. The book files are in "site" under the "tests" directory not in "content".
Otherwise I think for a quick overview its fine.
One thing I often find helpful is an example case in the way: Let's say you added feature x and want to add a test for it then you need to put the test at y in file z (and add dummy data to test with to directoryf/file w).
The reason this often helps me probably is that I get an idea of how the code is layed out and what the logic of this layout might be.

[choldgraf]

@BENR0 that's a great comment - I'll open a new issue with your suggestion so we can keep track of it!