Add some developer documentation

[AdrianDAlessandro]

Description

This PR slightly rearranges the README so that there is a separation between the standard installation and usage instructions and the developer documentation. It also tidies up the existing markdown files to pass almost all of the markdownlint rules that were previously ignored

Fixes #62

Type of change

• Documentation (non-breaking change that adds or improves the documentation)
• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass (eg. pytest)
• The documentation builds and looks OK (eg. mkdocs)
• Pre-commit hooks run successfully (eg. pre-commit run --all-files)

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added or an issue has been opened to tackle that in the future. (Indicate issue here: # (issue))

[Thomas-Rowlands]

Looks good thanks

[alexdewar]

Good cleanup 👍