Document Notion integration

[andymeneely]

I've been really enjoying using Notion for my gaming brainstorming. I've even set up some simple CSV downloading so I can put my data straight into Notion databases and then Squib will download.

Now, I don't really see a reason for there to be integrated Notion support - except maybe some sort of DataFrame integration. But, we could definitely document some examples:

• Download CSV from database, without API token (easiest)
• Use the Notion API (via the Ruby gem) to:
  • Download a database and load up a dataframe
  • Save previews of cards back to Notion
  • Have Notion notify Squib when to update? Haven't see how to do that, but it would be nice not to have to poll.