Give admin the ability to edit about pages

[ori-near]

Problem
Admins are unable to directly edit the about pages on DevHub, Events, and Infrastructure Committee portals. Currently, they have to rely on developers to modify the smart contract.

User Story
As an admin, I need an easy way to update the about page without relying on developers to change the code.

Acceptance Criteria

• When an admin clicks the Admin page, they see a "About Page" tab in the navigation
• The about section shows the rendered about page content with a settings configuration button
• Clicking on the edit allows the admin to edit the content using markdown
• When the admin clicks saves, the About page content is updated in the about section + the actual about page

Note: The interface for this can be super simple and leverage our existing patterns.