WIP: Add details on how to update section, stage, and group information

Why is this change being made?

Multiple MRs have added new team members to group-specific roles without a matching entry in _categories-names.erb.

That entry is needed, otherwise the name is displayed on Product stages, groups, and categories in brackets and without the expected link to the person's profile on the GitLab team page. (Example here.)

The page previously only contained a link to https://about.gitlab.com/handbook/marketing/website/#working-with-stages-groups-and-categories which only covers stages.yml and categories.yml, not the further information we need for those who intend to update what's displayed on Product stages, groups, and categories.

This MR:

• Documents all applicable source files and when to use them, adding references to:
  • https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/master/source/includes/product/_categories.erb
  • https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/master/source/includes/product/_categories-names.erb
• Splits the Changes section into "Governance" vs. "Source files".

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