Add release process

[duncandewhurst]

It should include at least:

• replacing branch/version placeholders
• updating CoVE, including checking for broken links

https://ocds-standard-development-handbook.readthedocs.io/en/latest/standard/technical/deployment.html has some steps that we should do, too

[lgs85]

Noting here that the handbook is currently missing guidance updating the canonical schema reference on the docs pages. Also need to add some guidance on updating latest branches etc on RTD