Docs cleanup

[nicknisi]

Type: Doc updates

The following has been addressed in the PR:

• [ ] There is a related issue
• [ ] All code has been formatted with prettier as per the readme code style guidelines
• [ ] Unit or Functional tests are included in the PR

Description:

This updates the docs in dojo/core to be more readable in the doc viewer on dojo.io.

Changes

• Clean up headers to better fit in dojo.io navigation
• remove unused/outdated/empty docs (math, array, DateObject)
• change examples paths from src/ to @dojo/core/