Embed contributors counts, clarify terms

Why is this change being made?

There is some recurring confusion about the number of contributors. This MR clarifies how we count contributors and shows total contributors vs. wider community only.

Direct link to review app: https://patch-19826111638.about.gitlab-review.app/handbook/marketing/community-relations/code-contributor-program/index.html#number-of-contributors

Does this MR meet the acceptance criteria?

Conformity

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