Improve clarity of setting a subsection as graded

[jswope00]

In the Graded Subsections content, there is clear information about how to set up your grade gradebook and your grade ranges. But the final step of setting subsections as graded subsections is buried in a link that also talks about other subsection settings.

This relevant parts about setting a subsection as graded should be extracted and added as an explicit step in the grading process.