Change docs tasks to use environment vars over rake params
Change docs tasks to use environment vars over rake params

- Makes it much easier to use in CI scripts and so on. As well as
  passing arguments to multiple, chained, tasks. The following
  environment variable now exists:
  - DOCS_CONFIG_PATH: Specify the path to the Jazzy config file.
  - DOCS_PUBLISH_REPO_URL: The URL of the repo used for the published
    docs.
  - DOCS_PUBLISH_REPO_BRANCH: The branch in the publish repo that
    should be used for the docs.
- Improves the grouping of helper functions in the tasks file as well.

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