Premium organizations can add custom sections to the Circle pages of their circles. This power is granted to each circle's Secretary, Lead Link, and all the organization's Admins.
Go to any of your Circle pages and look for the Manage custom settings button at the bottom of the left sidebar. Click to reveal a new screen that allows you to add new sections to the sidebar of your Circle Page.
This can be used to post links to further details on your purpose and other governance, or for other content that you wish to highlight.
You may have to reload the page after making changes in the custom sections in order to see the changes on the page.