Document how to define a custom environment, eg. for CI

[roborourke]

The configuration defaults are currently tailored to our stack and the local environments and doesn't properly account for cases where a custom environment and set of overrides would be needed, for instance on CircleCI.

It's possible to define the constant HM_ENV_TYPE and use that value in the config environments section so we should document how to do that.

Props to @Nikschavan for raising this.

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Acceptance criteria:

• Add a section to the configuration.md doc in the getting started guides titled "Defining a custom environment"
  • Explain the use cases eg. changing config for a CI environment
  • Add an example of defining HM_ENV_TYPE in the .config/load.php if any CircleCI or Travis CI environment variables are found
  • Show an example config using this custom environment to turn off certain features