Analytics documentation links are jumpy

Problem to solve

In the documentation, the Analytics navigation menu is "jumpy" on the Analytics index page, because the section categories listed in the navbar are all links to different sections of this single index page. See the video here: gitlab-docs!1248 (comment 432231483)

Proposal

Create separate pages for the different Analytics section categories:

1. Instance
2. Group
3. Project

Note: Technically this is a gitlab-docs issue, but is being created here to avoid losing track of the issue.

Screenshot