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

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