Document the Media Library module

Problem/Motivation

We need to write documentation for the Media Library module.

Proposed resolution

When #2962110: Add the Media Library module to Drupal core is closed we should write a new documentation page for the Media Library module, which will need to be expanded as the MVP gets closer (see #2834729: [META] Roadmap to stabilize Media Library).

Remaining tasks

Wait for the parent issue to close, work on docs.