Set up optional color inversion for docs images

[leburgel]

Sets up optional color inversion for images in the docs. Seems to work fine for both .png and .svg images, by including images through raw HTML and adding a color-invertible class:

```@raw html
<img src="path/to/image.svg" class="color-invertible"/>
```

That way images are not inverted by default, which seems appropriate for e.g. plots. This also allows for specifying the size by adding something like width="50%", so I think this is sufficient for our use cases.

Fixing the logos to only have one image file was a bit more tricky. In particular, the only way I managed to make the README logo invertible was to embed some custom css into an svg and then including the normal logo through base6464 encoding. A little shady, and if the logo changes this README logo would have to be manually updated. But I can't really see a way around this.

I was also planning on updating and adding some actual images, but I'm not sure when I'll get to that so it might be good to merge just the setup first.

Preview is here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

see 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[lkdvos]

Thanks, looks good to me. I like that it is optional, and that the required setup is pretty minimal.