Update the fe documentation to specify the usage of d3

What does this MR do?

Updates our FE docs to specify that we will be using d3 as part of our efforts to make GitLab more maintainable (graphs wise)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Review
  • Has been reviewed by Frontend

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/42736