Outstanding setup tasks

[duncandewhurst]

• Add a license
• Add a changelog
• Make repository public
• Build on ReadtheDocs
• Set up subdomain (json-schema-extension.opendataservices.coop) and redirection
• Add a link to the built documentation from README.md and the website field in GitHub
• Update documentation with links to relevant sections of OS4D, once updated

[duncandewhurst]

• Set up subdomain (json-schema-extension.opendataservices.coop) and redirection

See Planio-53838