docs: add codespaces section to development guide

[jpd-defra]

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Description of Changes

Added small section on using a Codespace to create a dev environment.

(No worries if you don't think it's useful and want to reject!)

Link to Issue

#283

PR Checklist

• [x] The PR title follows the conventional commit standard
• [x] There is a associated GitHub issue
• [ ] If I have added significant code changes, there are relevant tests
• [ ] If there are behaviour changes these are documented