Refactor README, introduce development guide

[nicholasjhenry]

This is the first PR in a series to improve documentation and the onboarding experience for new developers.

• Simplifies the README.md to provide an overview of the project without overwhelming the reader.
• Adds a development guide to get new developers set up (based on my experience of setting up the project).
• Adds an overview guide extracted from the README.md.

No information was lost from the original README in this documentation revision, just extracted to other files and reformatted.

NOTE: the development guide does not cover running the server locally to develop with ink-library. This will come later in another PR. This only gets a successful test environment running.

[nicholasjhenry]

@Marie000 Do think you will have time to look at this today, please?

[nicholasjhenry]

@Marie000 Is this good to merge?

[nicholasjhenry]

@Marie000 any update on this?