docs: update docs and some of the scripts that run them

[samrose]

Problem

running dbmate tool resulted in inconsistent outcomes due to the nature of how it was run

Solution

• Updated the docs to reflect the correct steps
• Adjusted devshell to handle creating the environment with the right DATABASE_URL value
• adjusted dbmate tool to strip leading insert statements (if present) so that it doesn't need to be run twice in order to produce consistent outcome.