[Docs] DataFed Metadata Mode Documentation

[AronPerez]

Problem We Want to Solve

As a DataFed user, I want to understand how Metadata Mode works so I can use in

• in the browser
• in the Python CLI

Describe the Solution You'd Like

Clear and concise documentation around how to use DataFed Metadata Mode as a user/admin

• in the browser
• in the Python CLI

Acceptance Criteria

GIVEN a user/admin wants to know how to use Metadata Mode
WHEN the user looks at documentation
THEN they read the documentation
SO THAT they can use DataFed Metadata Mode