[Feature Request] Support module, skill, and phrase comments

[LoveIsGrief]

Some modules, skills, or phrases could do with comments from the course creators. Most obvious would comments about grammar.

There are two ways to achieve this:

• an inline comment field in the YAML that takes a markdown string
• a comment_file field that takes the name of a file at an expected path containing the markdown

Using markdown will allow the use of images, tables, and basically everything that markdown supports.

[kantord]

I'm curious: how would you display these comments to the users?

I like the idea, though I think I am against adding the possibility to include features and longer text, for 2 reasons:

• there is already a feature to display a large markdown document with explanations for a skill
• displaying long text to the user while practicing a long skill could disrupt the flow of quickly practicing a lot of examples.