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 https://github.com/cct-datascience/workshop-wednesdays/pull/43#discussion_r1446305621_