Move docs

• Migrate deployment docs to a new sub-article.
  • Check how relevant they still are (ask the author, @jchristgit)
  • Deployment docs are low priority.
• Migrate configuration docs to a new table in the environment variables section.

Tests

• Document how to run tests.
• Possibly document how to write tests. See python-discord/bot#716 for more details.

Migrations

• Document how to create migrations and when new ones are necessary.
• Document how to name migrations well rather than using the automatically generated names.
• Document how to deal with conflicts i.e. new migrations from upstream.

Other

• Document how to re-build the Docker image in case dependencies are changed.