Make optional the use of mathjax

[benoitdudu]

Hello,

Thanks for the great job you made with PeakRDL.

I work in an environment where I do not have access to the internet. The loading of the generated html documentation is very slow due the fact that my browser try to access to the mathjax js library (https://github.com/SystemRDL/PeakRDL-html/blob/main/src/peakrdl_html/templates/index_base.html#L21).

Do you think it would be possible to make it optional?