Improve docs

[JostMigenda]

Several improvements to the documentation pages and README:

• Improved instructions for how to contribute to and cite SNEWPY in the docs pages
• README now links to the docs page on how to contribute, rather than duplicating part of that content.
• Updates the README file with short sample code snippets to be more welcoming to new users

Thanks to @santosmv for suggesting the sample code snippets!