Guide about what requires downtime

What does this MR do?

This MR adds a guide describing various SQL operations and whether they need downtime or not.

Are there points in the code the reviewer needs to double check?

Spalling and styling of the text mostly.

Why was this MR needed?

Developers aren't always aware of the impact of certain operations and the documentation of MySQL and PostgreSQL can be quite confusing at times.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together