Guidance on how to set a release date

[yolile]

The current documentation only says what the release date is, but we need guidance or worked examples on how to set this field. According to #854 and other recent experiences with publishers, we know sometimes they struggle to set this field correctly.

The guide should cover at least the following:

1. Reinforce that the release date is the date when the data contained in the release was published and not when the release itself was created
2. How to choose the release date if the source data doesn't have that concept
3. How to choose the release date if the source date doesn't have dates at all for some procurement processes.

From Slack conversations with @jpmckinney and @fppenna:

Bad quality release dates: Don’t exclude data. A release with a bad date is better than no release. If they really have no idea (not even a year), then omit the date field.
No release date concept: Use either the tender start date, the award date, or any contract stage date (in that order of priority) as a proxy for the release date. That order of priority should be reversed since the first time contract data was published is e.g. on the dateSigned or whatever – it’s not on the tender start date (at which time the contract data doesn’t exist yet). Otherwise, this is a good enough proxy for the real date.