Add section on branches, tagging and deployment

[duncandewhurst]

We're reusing the approach from OCDS:

• https://ocds-standard-development-handbook.readthedocs.io/en/latest/standard/technical/repository.html?highlight=branch#branches-and-tags
• https://ocds-standard-development-handbook.readthedocs.io/en/latest/standard/technical/deployment.html#merge-the-development-branch-onto-the-live-branch