docs: update to new logo (Unstructured-IO#1937)

### Summary

Updates the docs and README to use the new Unstructured logo. The README
links to the raw GitHub user content, so the changes isn't reflected in
the README on the branch, but will update after the image is merged to
main.

### Testing

Here's what the updated docs look like locally:

<img width="237" alt="image"
src="https://github.com/Unstructured-IO/unstructured/assets/1635179/f13d8b4b-3098-4823-bd16-a6c8dfcffe67">

<img width="1509" alt="image"
src="https://github.com/Unstructured-IO/unstructured/assets/1635179/3b8aae5e-34aa-48c0-90f9-f5f3f0f1e26d">

<img width="1490" alt="image"
src="https://github.com/Unstructured-IO/unstructured/assets/1635179/e82a876f-b19a-4573-b6bb-1c0215d2d7a9">