Use transparent background for yugabyte logo on GitHub

[jaki]

The yugabyte logo on the main page of our github (i.e. referenced from the top-level README) has a white background. This stands out a lot when using github dark mode. Make it a png or vector graphic (e.g. svg) with a transparent background. Also try to make the image more crisp.

[jaki]

Making the background just transparent might make it difficult to read the blue text. How about also adding some white glow or border around the edges so that

• on a white background, the border doesn't show
• on a dark background, the border shows and makes it look clearer

Might want Grishma to help create that.

[jaki]

The image should reside somewhere external, like yugabyte.com. Committing large images to repositories is bloat. See these examples:

• https://raw.githubusercontent.com/keras-team/keras/master/README.md

  ![Keras logo](https://s3.amazonaws.com/keras.io/img/keras-logo-2018-large-1200.png)

• https://raw.githubusercontent.com/tensorflow/tensorflow/master/README.md

  <div align="center">
  <img src="https://www.tensorflow.org/images/tf_logo_social.png">
  </div>

• https://raw.githubusercontent.com/ohmyzsh/ohmyzsh/master/README.md

  <p align="center"><img src="https://s3.amazonaws.com/ohmyzsh/oh-my-zsh-logo.png" alt="Oh My Zsh"></p>