Add + support image alt text and captions.

[mattstein]

Description

Adds imageAlt and imageCaption fields to be used within Content Blocks for articles, so we can mention custom Asset field layouts in the guide.

Fields in layout:

Example caption:

Markup with caption:

<figure>
  <img class="w-full" src="(...)" srcset="(...)" width="380" height="152" alt="Two bicycles perched in a tree" sizes="(...)">
  <figcaption class="px-6 py-3 text-gray-600 text-sm text-center">Two bicycles perch precariously in a tree.</figcaption>
</figure>

Markup without caption:

<div>
  <img class="w-full" src="(...)" srcset="(...)" width="380" height="152" alt="Two bicycles perched in a tree" sizes="(...)">
</div>