Add missing links to resources

What does this MR do?

I noticed that the API resources list: https://docs.gitlab.com/ee/api/README.html#resources didn't point to Avatar API: https://docs.gitlab.com/ee/api/avatar.html

MR adds links to any resources not currently pointed to (there was more than one) and makes other minor improvements to the section. I assume there isn't a reason to not point to these resources from the main page.

Related issues

EE port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23605.

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards