Improve docs and tidy up codebase

[mzabani]

After announcing codd in Discourse and Reddit, two important things happened:

• One person did some digging in the codebase and was confused by legacy SQL migrations that no longer make sense.
• In the Reddit announcement, two different people asked for comparisons with existing tools, and one of them asked for an explanation of how codd applies migrations.

I should address these items.