Add Images to README.md

[vexx32]

PR Summary

Prettifies the README with some screenshots to add some colour.

Context

Readme files should be pretty. 😊

Changes

1. Replaced the result examples in getting started with screenshots from the relevant states of completion (0 completed koans -> 1 completed koan)
2. Tidied up links extensively and moved all links to the named reference section at the bottom so it's easier to keep links up to date.

Checklist

• Pull Request has a meaningful title.
• Summarised changes.
• Pull Request is ready to merge & is not WIP.
• Added tests / only testable interactively.
  • Make sure you add a new test if old tests do not effectively test the code changed.
• Added documentation / opened issue to track adding documentation at a later date.