Non-existent SettingsType referenced how-to documentation

[rgraber]

The how-to for creating a new plugin app involves using the SettingsType enum, which does not exist. We should either add the enum or remove the references to it in the documentation.

A/C

• [ ] The docs should no longer include a reference to the non-existent edx-django-utils.plugins.constants.SettingsType enum
• [ ] It should be clear that other IDAs may have other constants in use and that it's the responsibility of the plugin creator to use them appropriately

[rgraber]

Replace SettingsType.PRODUCTION and SettingsType.COMMON in the documentation with the actual strings they refer to and reference that they can be found in edx-platform here: https://github.com/openedx/edx-platform/blob/599d5d054cd43b3ec7a291e38de96108dfa28e14/openedx/core/djangoapps/plugins/constants.py#L25 . Clarify that this is an example from edx-platform but other services may have their own strings set up.