Outline for Activity Tracker module guide

For context see #2887480: Outline, standardize, and complete documentation for Drupal 8 core modules.

Proposed outline

Here's a proposed outline for what the documentation for the guide at https://www.drupal.org/docs/8/core/modules/tracker should contain.

• Page: Activity Tracker module overview
  Module description, and example use cases. What does this module do? When should I enable it?

  Start by copy pasting from help text displayed by the help module at admin/help/tracker, and adjust

• Page: First steps
  Documents typical next steps when using the module, configuration, terminology, and concepts.
  • Pages created: "Recent content" & "Activity" for each user profile.

Next steps

• Review above outline and suggest changes
• Review and update existing overview page
• Draft pages in guide and publish for review