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"/>
```

This also allows for specifying the size by adding something like style="zoom: 180%".

I added some images to the boundary MPS and 3D partition function examples by way of example.

I also updated the README, since we're probably at the point where we should no longer be preemptively covering ourselves for unanounced breaking changes.

Preview is here.

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After the build completes, the updated documentation will be available here