Switch the language spec to Lkt/lkm, the interpreter to Python3 and
recent generated Python APIs, and update explanations to accomodate the
evolution of Langkit in general.
This patchs updates the langkit tutorial, starting from the `Getting
Started` section. It consists of updating the new file/path names used
by langkit when creating a project and switching some Python code from
version 2 to 3.
It also adds a new file, `kalint.py` to help the reader to correctly
assemble all the pieces of the langit toy interpreter.
Currently, code blocks are sometimes annotated with the "::" construct
while others are annotated with the ".. code-block:: <language>" one.
Turn all of them into the latter form so that GitHub can highlight them
correctly.
Change-Id: I9e653d21a153cdf96a6ee6e4fa9e55e622170c03
TN: P111-033
