Add User examples to docs

[mdtanker]

Description of the desired feature:

It would be good to have a section of the website which lists examples of people using PolarToolkit in whatever means, such has papers, presentations, or teaching.

• add examples section to index.md
• add docs/examples/ folder
• add index.md file to examples folder
• add section of contributing guide on how to add examples
• add section to ReadMe and homepage asking users to add examples