Re-structure Analytics documentation index

What does this MR do?

Previously the Analytics documentation index was built around the Analytics Workspace concept. It did not sufficiently explain features located at the group and project levels, outside the instance-level Workspace. Plus, we are moving away from the Workspace concept anyway. This re-structures the Analytics documentation index into 3 sections, focused on the perspective/scope of the features:

1. instance (Workspace)
2. group
3. project

The Analytics features available at each level are listed.

Note: This also provides a place to link to the upcoming Repository docs.

Screenshots

Previous Analytics documentation index:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content