Visual Testing Tutorial

Write a detailed introduction to the image differ and live image added in #284.

Come up with some interesting examples, perhaps adding a watermark to an image and building a custom control for PySide6. Look for most similar pairs of Chinese characters?

Talk about sources of fragility and assembling layers of building blocks to keep the tests readable.

Start by reading the image differ and live image work added in #284. Build the tutorial around examples such as watermarking an image, a custom PySide6 control, and similar pairs of Chinese characters, then explain fragility and how layered building blocks keep tests readable. Done means the visual testing introduction is detailed and covers these examples and concepts.

Written by the indexing model from the issue text.