Improve documentation

[leon-vv]

Create a directory generate_docs/ which contains code to generate documentation. To generate the documentation, use bash ./make_docs.sh. The version number has been bumped to v0.9.0rc1 and documentation for this version has been commited. Open docs/docs/traceon/index.html in a browser to see the current state.

[jobdewitte]

Nice work, I like the overall feel of the documentation and usage also seems quite intuitive. I do have a few remarks and suggestions.

• Is there a way to make the classes and functions appear as a dropdown from the modules instead of on the bottom of the page? This would feel more natural than having to scroll down everytime, especially as the project grows.

• A few of the longer class method names, especially in the Exitation module (e.g. electrostatic_potential_at_point are too long for the side bar on my 15.6'' laptop and have a single 't' on the next line. This feels a bit unprofessional? Is there a way to dynamically adjust this side bar? Otherwise I would suggest making it a bit wider.

• I would prefer a slightly more compact version of the documentation, which makes it a bit faster to navigate between functions. Maybe this is as easy as adapting the CSS to scale down the fontsize and whitespaces between functions.

• It would be nice to have code blocks that can be copied.

• More content related: some sort of general overview of the documentation, a how-to-start guide, explanation of the differences between Traceon and Traceon Pro etc. should also be included in the docs, but I feel that is beyond the scope of this PR.

[leon-vv]

I would like to merge this so that we can move to the next release. I propose a new issue to collect your points which can not be tackled quickly: #180. Regarding your points:
(1) Quite difficult, would imply remaking the whole template (time intensive)
(2) Fixed
(3) Added to issue
(4) Added to issue
(5) Added to issue (I finished the Einzel lens example)