Update contributing to documentation page

Why is this change being made?

Update the documentation page to add further clarification on when to follow branch naming conventions and add Charts to the list of documentations.

Closes https://gitlab.com/gitlab-com/marketing/community-relations/contributor-program/general/issues/35

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