Docs: improve discoverability of image compression

[emilk]

What

We often talk to customers that are unaware that we support logging encoded/compressed images.
So I added some docs for this (in 3096c17).

This became a 🐰🕳️ because the doc tags (\py etc) didn't work as expected: they would put all language-specific stuff at the end, but in this case I wanted it interleaved into the rest of the docs. So I fixed that, with a lot of effort.
Now \cpp, \py, \rs acts like a simple per-line filter.

In the meantime I removed some unused code, making everything shorter in the process.

Checklist

• 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 the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
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[github-actions]

Size changes

Name	main	5675/merge	Change
rrt-star.rrd	8.16 MiB	5.78 MiB	-29.17%