Documentation of handling of long-range corrections

Description

This MR adds an example in the "Advanced Topics" section that demonstrates how to handle long-range (Coulomb) contributions to the second-order force constants. The example is provided in the form of a jupyter notebook. The MR includes several smaller changes related to the introduction of a jupyter notebook based example.

• new job in the CI: test_notebooks
• additions to the docker image: nbsphinx, nbmake (for testing) and pandoc
• removed unused files in the doc/figures directory as well as the doc root directory

Comments

• I was unable to find a way to add index entries corresponding to the jupyter notebooks. As a result, there are no entries related to "long-range corrections", "Born effective charges" or similar in the index. Note that it is difficult to search for information about "index", it being the default name of practically every starting page on the web. Still I carefully went through the documentation of nbsphinx to no avail. I also tried unsuccessfully to come up with a solution via the include directive.

Screenshot

Closes #322 (closed)