Readme Overhaul

[paulbrodersen]

Description

What is this PR

• [ ] Bug fix
• [ ] Addition of a new feature
• [x] Other

Why is this PR needed?

The current README is outdated and very terse.

What does this PR do?

I have made the following changes:

• Removed the old images as they had a very low ink-to-information ratio; added an image from the paper with a high ink-to-information ratio as a placeholder. I don't think this particular image should be used long-term but it is better than the previous ones and can serve as a template for a new and improved gallery image. However, I lack both time and the data to make one, so someone else would have to step in.
• Removed outdated badges (GPLv3; Black); added a new "Downloads" badge as "social proof" (corny, but so are all badges and it helps).
• Added a "Quickstart" section demonstrating the basic workflow.
• Added a BibTeX entry to the "Citing BrainRender" section.
• Added additional context to the "Documentation" section (who or what is BrainGlobe and why does it host the BrainRender documentation?).
• Shortened the tag line, so it's a tag line and not an essay.

The result can be inspected here.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.47%. Comparing base (1ff1a5e) to head (fc2c71e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #337 +/- ## ======================================= Coverage 86.47% 86.47% ======================================= Files 27 27 Lines 1220 1220 ======================================= Hits 1055 1055 Misses 165 165 ```

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