Improve the documentation website for interpret-community and update the main readme

[imatiach-msft]

Improve the documentation website for interpret-community repository and update the main readme on github.
Specifically:

• Move more specific installation instructions to the website, simplify instructions in readme. Update the instructions for pytorch. Fix the grouping which is currently wrong.
• Move section about API docs up, and change it to documentation section
• Move example notebooks links from readme to documentation website. Add more links to new notebooks (which we somehow missed adding when they were committed).
• Add some missing sections to API docs
• Add supported models and explainers page to documentation website
• Update index page to include more links to the new repositories and websites
• Add overview page with detailed installation instructions to website
• Fix broken k-means function documentation which was throwing error in sphinx build