[web] Benchmark gallery

Restructuring of documentation sections:

• each category (e.g. Steady State Diffusion) in each sub-documentation (e.g. Benchmarks) is now a proper Hugo section
• sidebar navigation is now fully based on these sections, therefore no need for superficial menu entries in each page
• sidebar is now independently scrollable
• Benchmarks section now has:
  • An overview gallery over all sub-categories, clicking on a sub-category now shows
  • another overview gallery with pages (benchmarks) in this category (image-parameter in page frontmatter for selecting the shown image, same parameter also on sub-categories)
  • breadcrumb navigation
  • Jupyter benchmark to web site conversion adapted to new structure (image-paramter in frontmatter as well, given in the first cell or first image is selected per default). Benchmark category has to be given as web_subsection-paramter.

I suggest to just review the preview (below) or individual commits.

--> Preview here (updated) <--

Other views as discussed in todays jour fix can (and will) be added later with tagging / other metadata.

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1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?