C++ Doc improvements

[Wumpf]

What

• Almost fixe_s C++ API Docs webpage #3974
• Follow-up of C++ Doxygen documentation & many doc improvements #4191

A few remaining items & polish for the C++ docs. Now uploaded to gh-pages as well on push to main (🤞 it works.......)

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• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

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[nikolausWest]

For this to actually close #3974, we also need to update the main website to link to the new hosted C++ docs. At least it should set up to do that on the next release. Is that included here? I didn't see it on a quick glance.

[Wumpf]

Not included yet, no. I don't know yet what to link to (see also unanswered questions on internal Slack and stuff I brought up on last standup). Right, I suppose we should keep the ticket open then until the release

[nikolausWest]

I suppose we should keep the ticket open then until the release

I don't think we need to keep the ticket open until release. It should be enough to keep it open until the release is prepared somehow such that we will have links to the new docs after the release (if that means just updating the release checklist that would be OK)