Add examples to the docs

[richski]

Small PR that:

• Adds CodeSandbox URLs for each demo in the README (except for the CustomSourceNode).
• Adds a new AdvancedExamples.md for any future advanced usage examples.
• Cleans up the README - table of contents, typos, formatting, etc.

[richski]

...what do you think about putting the codesandbox link before the code snippet?

@PTaylour Yeah that's better - done https://github.com/bbc/VideoContext/pull/165/commits/fa338c8d7227f40f2e296f62bf3d7bd2cf4edfb6