Add docs for celeryviz

[bhavya-tech]

Since official release is close, it is important to start ideating on docs for the whole CeleryViz.
We have 3 main components:

1. celeryviz python library
2. celeryviz_frontend_core flutter widget
3. celeryviz_with_lib for building webapp to be bundled with python library

We need to plan docs for all three major components. The third one might be very small one.

Points to consider:

• Should we keep a common doc for all different components?
  • If yes, how will they be updated with each changes of PR?
  • If no, what would be best way to manage multiple docs?
• Where will they be hosted?
  • GitHub Pages seems best option.
• Which docs library is best suitable for this project?
  • Docusaurus has clean UI and can be built easily with Markdown.
  • However, most of the Python official docs are on Read the Docs using Sphinx
  • For flutter docs, we have the official dart doc which can be used too.