Improve and document development environment setup

[jcoglan]

Hi, this is James from @neighbourhoodie. I have been working on STF milestone M1 to improve the development workflow and documentation, so that new contributors can get started quickly and successfully.

To meet this goal I have made the following changes:

• Add make commands for starting/stopping/destroying the Docker-based dev environment, running migrations, creating an "admin" user, and displaying the routes.

• Remove manage.py, which would not load due to its use of flask-script, which has not worked since the release of Flask 2.0. (It tries to import flask._compat which does not exist.)

• Document the newly added make commands and remove the outdated documentation which has barely been changed since the project's inception.

[jcoglan]

@ewdurbin I've addressed the comments you left, save for the issue of non-container dependencies.