Define JTBD terminology

Why is this change being made?

We use the JTBD framework, but this is a vague framework used different ways in different organizations. I think having some shared terms and definitions will help everyone understand how we use the framework at GitLab.

How will this merge request impact page speed?

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what