The docs do not make it clear what features should go in Tutor core vs plugins

[regisb]

@fghaas made a comment here:

It looks like your policy is, by and large, "don't add new configuration settings to Tutor, use a plugin instead." That's all fair and good but please make that policy explicit. Currently, the contributor documentation does not mention any of this. It's not at all self-evident that people should default to not adding new configuration parameters.

We need to make it clearer to external contributors which changes should go in Tutor core, and which ones should go in plugins.