Add session to generate screenshots from html

[Spacetown]

The used tool wkhtmltoimage was hard to get running on my Mac and I discovered that the tool can't handle CSS variables. The tool isn't developed any further and I searched for something else.
This PR adds a session to generate a JPEG from a HTML by using a server running a Chrome in a headless mode. The server needs to be started manually by the user if it's not started yet. The server wasn't able to provide PNG therefore I switched to JPEG.
I updated the docs and the examples that they fit together.

[Spacetown]

@latk Please can you check if this dependency is OK? I want to get rid of the manual capturing of the HTML for the documentation. Maybe we can try to capture the screenshots during the build of the documentation, if there is nothing changed in the HTML the file is binary identical.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1c0e1d5) 95.27% compared to head (2555c61) 95.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #877      +/-   ##
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+ Coverage   95.27%   95.30%   +0.03%     
==========================================
  Files          54       54              
  Lines        4525     4534       +9     
  Branches      893      893              
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+ Hits         4311     4321      +10     
  Misses        130      130              
+ Partials       84       83       -1     

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[latk]

if this dependency is OK?

sure, wkhtml2pdf seems obsolete, and https://github.com/bedrockio/export-html looks OK (open source, easy to use via scripts)

The server needs to be started manually by the user if it's not started yet.

Not the biggest fan of this because that's a manual step. We already automatically build and launch Docker containers for tests, so starting the Chrome container automatically could make sense as well.

The server wasn't able to provide PNG therefore I switched to JPEG.

That's tolerable if there's no other way, but nice crisp PNGs would be nicer. The container's docs mention PNG export – what challenges did you encounter when using that feature?

if there is nothing changed in the HTML the file is binary identical.

If that's the case that's great. Previously, the screenshots would include metadata like the creation date (I think), which made it impossible to check if the screenshots were up to date.