Write an introduction for each section

Sections like 2D, 3D, Audio… only feature a list of links on their index page. We should introduce and organize the content in each of them. We can split table of contents into multiple too. See the Optimization page for an example or Contributing after #4067 gets a merge.

To me, the introduction should briefly explain the content a user can find in the section, and link to other relevant sections or pages. For instance, if we have a Rendering section, it could feature a paragraph like :

> This section focuses on using rendering features. To learn about rendering performance optimization, head to:ref:`optimization_ref`.