Change homepage introduction to rich text

[thibaudcolas]

We use the website to occasionally promote things to our audience. For example Wagtail Space, or getting feedback on specific aspects of the project.

It would be nice if we didn’t have to hard-code homepage content for this. Update the existing introduction field to support rich text:

guide/apps/core/models/home.py

Line 15 in 8b0f9d0

introduction = models.TextField(blank=True)

As part of implementing this, it would be nice if there was a text that the field’s rich text content does render as expected.