Add guidance on when to advertize workshops

[Aariq]

For emails and other announcements (such as on Slack, Mastodon, etc), what's the timing of the announcements? For example:

• within one week after workshop date finalization, send announcement
• 3 weeks before workshop, send reminder
• etc

Originally posted by @Chris-Schnaufer in cct-datascience/workshop-wednesdays#43 (comment)