Add table of contents to HTML report

Some diffoscope results are huge, thus suggest to having 'table of contents' to easy navigate the results, can be implemented using CSS only.

for example currently results are like:

https://mushiyo.github.io/pandoc-toc-sidebar/outWithoutTOC.html

but will be better with sidebar like:

https://mushiyo.github.io/pandoc-toc-sidebar/outWithTOC.html#variables-set-by-pandoc

or it can use floating TOC such as:

https://stackoverflow.com/questions/12232675/how-to-make-table-of-contents-floating

or (which I prefer) the sticky version (auto-show related item in TOC when related diff section is shown):

https://ma.ttias.be/adding-a-sticky-table-of-contents-in-hugo-to-posts/#whats-the-sticky-table-of-contents