Update pr-functions vignette to use 'main'. Fixes #1491.

[rkb965]

Hi,

First, thank you so much for the guidance on this vignette — I really appreciate the help and the opportunity to contribute.

This PR makes a few updates:
1. Changes references from master to main.
2. Updates the second round of review to show the reviewer suggesting a change via GitHub (instead of fetching and modifying locally). This removes some detail about the reviewer’s workflow.
3. Adds Garrick’s diagram to summarize the pr_*() flow.
4. Updates all screenshots to include both ...-light.png and ...-dark.png, so that if/when vignettes move to Quarto, the dark theme will be supported without re-creating the images.

There still may be small errors — please let me know if you spot anything and I’ll be glad to fix them.

Thanks again!