Enable MathJax for documentation

[AlexRTer]

This PR closes #673.

The current online documentation is generated using mkdocs with the material theme.
Thus, MathJax can simply be enabled in mkdocs.yml and needs to have the corresponding JavaScript runtimes linked.

This PR adds the default setup suggested at https://squidfunk.github.io/mkdocs-material/reference/math/
without further customization (instant loading is not enabled in mkdocs.yml so the corresponding section in mathjax.js has been left out for now).

MathJax allows for TeX expressions to be rendered properly in the online documentation using $...$, \(...\) for inline- and $$...$$, \[...\] for block-syntax. Please note that the block syntax requires to have the enclosing $$ on separate lines each (https://squidfunk.github.io/mkdocs-material/reference/math/#using-block-syntax).
On Github, most of these expressions are rendered properly. There are some exceptions, e.g. spacing $... \, ...$.

There are also a few minor changes to improve consistency and fix some formatting errors in the docs.
I attempted to clarify the documentation for ctables, please change this back if you deem it unnecessary.

[philipportner]

LGTM and the output is now nicely displayed. Thanks for the fix @AlexRTer !