"Getting started"

[MichaelClerx]

• Replace "Using PINTS" in github readme with something longer, and called "Getting started"
• Ensure this works with github dark theme (watch our for transparent PNGs!)
• Make sure it looks good on https://pypi.org/project/pints/
• Rewrite the first few sentences of the API docs, to make sure they explain that:
  • the docs are API only, don't explain how to use
  • there is a "Getting started" on github (README)
  • there are examples for every method, plus several intros, in github/examples
  • installation instructions are on github too