Update docs URL and set up redirect

[duncandewhurst]

https://docs.opendataservices.coop/projects/json-schema-extension/latest/

• Read the Docs redirect from https://json-schema-extension.readthedocs.io/
• GitHub repository website field
• README.md
• Schema id fields
• Documentation pages
• OS4D
• Links from OCDS documentation (see Rewrite docs/development/schema.md and related components and patterns os4d#49)