Use flexible layout for hlist directives

[Blendify]

Problem

The current hlist can look bad on mobile or when using many columns.

Reproducible Project

https://docs.blender.org/api/current/bpy.types.AlphaUnderSequence.html

Error Logs/Results

As you can see the list items extend beyond viewport when the viewport is narrow.

Expected Results

The columns should break into new rows.

Environment Info

• Python Version: 3.9.x
• Sphinx Version: 3.5.1
• RTD Theme Version: 0.5.1

[Blendify]

This is relatively easy to fix using display: flex and flex-flow: row wrap