Draft: Introduce a new section for tutorials, and a first tutorial

This commit introduces a first tutorial in a newly introduced new "tutorials" section. These tutorials are planned to explain how to solve particular PDE problems with particular sets of Dune modules.

The tutorials are implemented as pairs of a markdown file with a C++ or Python file. The markdown file contains the text, and includes parts of the source code that are marked by two specific keywords (START-SNIPPET and END-SNIPPET).

This first example explains how to solve the Poisson equation using Lagrange finite elements and the dune-functions module. The code is taken from the 'examples' section of dune-functions. The text consists of little more than placeholders at this point.

This MR is marked as Draft for a number of reasons:

• There may be a better way to make hugo create the new 'tutorials' section.
• I am not sure where to place the new files. What are the hugo rules?
• I am not sure how to best organize the new section. I want tutorials for C++ and Python, and they could be organized by level-of-difficulty, field-of-application, top-level Dune modules, etc.
• One would like to have automatic testing for the example code files, to prevent them from bitrotting. I have no idea how to achieve that, but it is beyond this MR.
• I am not sure whether having the example code files in the dune-website repo at all is a good idea. Several Dune modules have examples directories already. Maybe we should add the markdown files there, and make Hugo import them (just as we do for README.md).