Add table of contents to navigation pattern when ToC has been developed

## What component is this related to?
Table of contents & navigation layouts.

## Why do we need it?
The table of contents should be added to the side and top navigation layouts.

The guidance should also be updated (the guidance is already written and the areas that need adding are highlighted).

## Related open issues
[Link to other GitHub issues]

Locate the table of contents and navigation pattern implementations, then review the existing guidance where the areas needing additions are highlighted. Confirm how the side and top navigation layouts are represented and what the completed guidance should cover. Done means the table of contents is included in both layouts and the highlighted guidance areas are updated.

Written by the indexing model from the issue text.