Add mathjax to properly support math equations

• This adds MathJax v3 including some workaround from this blog post to make it work with Kramdown: https://11011110.github.io/blog/2019/10/17/mathjax-3-jekyll.html
• Mathjax can be loaded via additional_js in the header section of pages.
• I added some example equations in the sample post to verify the functionality.

@hueser93 @erxleb87 Please have a look.