Post-1.0 cleanup tasks

[ehmatthes]

These are important tasks, but they are not critical to a 1.0 release. They are great tasks for the 0.9 release through 1.0, if there are no new issues coming up from the 1.0 candidate.

• [ ] Adopt the policy that all open issues should be specific, and if addressed would be welcomed as a PR. Move all tasks in open issues that don't fit this description to a longer-term section of the docs.
• [ ] Implement an architecture that enforces structure of the PlatformDeployer classes. This is probably an ABC. Read a bit about composition and protols before making this decision, and consider a plugin approach as well.
• [ ] Use pytest to orchestrate integration tests.
• [ ] Clarify some of the use cases and more subtle benefits, ie "All configuration is contained in a single git commit". This makes it a great way to explore how a platform works.
• [ ] Rename dsd_env -> .venv.
• [ ] Use type hints where appropriate, particularly in the deployer classes.
• [ ] Move integration test docs to rtd.