docs: add PDF versions of SVG diagrams

[fingolfin]

This does not yet build a PDF manual, but makes us ready for that.

The actual conversion shall be implemented by @various in PR #2307, which will have to be rebased once this PR here is in. He can then worry about details like how we can enable/disable building the PDF, integrating it into the CI, etc.

Also, this PR should help @varuntrehan7 figure out how to deal with the SVGs in Nemo.jl

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Note that many tools for converting SVG to PDF don't cut it (e.g. Cairo's svg2pdf, or inkscape, etc.). The only general purpose SVG renders I am aware of are web browsers. E.g. the SVGs in this repo use a blur effect for shadows...

One can use a site like https://jsontotable.org/svg-to-pdf to do the conversion.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.15%. Comparing base (af4b20b) to head (1d1ed94).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2324      +/-   ##
==========================================
- Coverage   88.16%   88.15%   -0.01%     
==========================================
  Files         126      126              
  Lines       32047    32047              
==========================================
- Hits        28253    28252       -1     
- Misses       3794     3795       +1     

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