Document anatomy of a plugin

[kdmccormick]

Original Context

This issue is similar to https://github.com/overhangio/2u-tutor-adoption/issues/29, except it's about the structure of the source templates instead of the rendered Tutor root.

There are some things I found confusing when working on a plugin. Once they became clear, plugin development got a lot easier. If I didn't have other working plugins to use as reference, then I'm not sure I would have been able to figure these things out without reaching out for help:

• Only templates under $TEMPLATE_ROOT/myplugin are rendered. The docs do say this but I wonder why it has to be the case.
• Within a plugin's template dir, only build, hooks, and apps are rendered. Other files are excluded. The docs don't aren't clear on this.
• It is not clear on what should go in apps, although I infer it is for "any file you want to include in your docker image".

Could we clarify these, either via documentation or simplification of the plugin API?

New Context

@regisb will be proposing a Plugin API v1 as part of #32.

Acceptance

Ensure there are docs explaining the structure of a version-1 Tutor plugin. Should be part of the plugin developer guide.

[kdmccormick]

This is obsoleted by #32. There is still documentation work to do for plugins, but this issue doesn't capture what needs to be done.