Explain how we resolve the paths to templates and plugins

[hegemonic]

If you specify the relative path to a template or (I think) a plugin, we attempt to resolve the path relative to the following (in order):

1. The directory where the JSDoc configuration file is located.
2. The current working directory.
3. The JSDoc directory.

We should document this on the config page. But first we should verify that the code path for loading a plugin includes jsdoc/path.getResourcePath, the method that handles the relative-path resolution.