Expandable/drill down Table of Contents in online documentation

As a documentation writer I want the table of contents to be expandable so that I can provide an overview of the content to the users, and allow them to dive deeper into details depending on what they're looking for.

Acceptance Criteria: (presented as Scenarios)

Scenario 1: Drill down
Given the reader is on the first page
When the reader selects an entry from the table of contents
Then the topic for the TOC entry loads
And the TOC shows the TOC for the topic.
And the TOC does not expand the sub-topics by default

Scenario 2: selecting elements in the TOC
Given the reader is on a page
When the reader selects an entry in the table of contents
Then the topic for the TOC entry loads
And the TOC does not change

Scenario 3: Browsing the TOC
Given the reader is on a page
And the table of contents is present
When the reader clicks a navigational marker in the TOC
Then the TOC/navigation pane does change
And the main content does not change.