Improve documentation UX / DX

[JTS22]

• Equations are not rendered correctly, (Markus: related to new katex version, additional escape backslash required, see dpsim-docs for correct latex rendering)
• Although Hugo seems to support syntax highlighting via pygments, the code contained in the documentation is currently not correctly highlighted (Markus: we just need to mark the code blocks correctly)
• The doxygen-documentation seems to not be generated correctly: https://sogno.energy/dpsim/doxygen/index.html
• The Dockerfile included in the docs/hugo folder does build but does not include Go, so hugo does not work in there
• The docs cannot be built in the current Fedora34 DPsim-Dev container, because the versions of Go / Hugo in the repos are too outdated
• The actual markdown files are contained in many nested folders which do not directly correspond to the generated HTML page tree. This makes it hard to track changes, especially when most of the files are named index.html
• The links between pages only work in the built HTML version, but cannot be used to jump between markdown files on GitHub.

Some of these issues could be solved by using other tools better suited for building code documentation, e. g. mdbook. This results in a file structure similar to here: https://github.com/rust-lang/nomicon/tree/master/src

[m-mirz]

I am fixing several issues in #165

• broken latex code
• missing images
• mark code clocks correctly
• update dockerfile, gh actions is already updated
• remove folder / index.html level for some sections

The only issue that I do not know how to address is links between MD files that work in the repo AND the website.

@JTS22 I think that most if not all issues that we have in the documentation are not related to hugo/docsy. We just need to do some clean up work ;-)

[m-mirz]

@dinkelbachjan This feature could be interesting for the discussion about a new solver class structure: https://www.docsy.dev/docs/adding-content/diagrams-and-formulae/#uml-diagrams-with-plantuml