Add theory lesson docs

[jdwilkin4]

Just realized there were no docs for a theory lessons page.

Even though the staff adds lessons, there should be docs for that and how to update lessons. Especially when it comes to the interactive editor.

[majestic-owl448]

I could add it in #1244 ?
we can probably use various parts already present in the review page in that PR

[jdwilkin4]

It looks like that PR was already merged in. so this theory lessons one can be a separate PR.

[majestic-owl448]

do you want to do the lesson docs yourself?