Update GH PR Guide Page

Nobody has claimed this yet.

Further updates to the first two sections of the [GitHub PR Guide](https://imageomics.github.io/Collaborative-distributed-science-guide/wiki-guide/The-GitHub-Pull-Request-Guide/) are still needed to align with [GitHub's recent updates](https://github.com/orgs/community/discussions/163932), though this is more about updating screenshots than actual content. There are a few more features that may be worth highlighting, but a major overhaul is not required.

One more thing to note in that section is actually picking a reviewer or reviewers, i.e., that it should be those that are most familiar with the content after the author and that they must have write access to the repo for it to satisfy branch protections (arguably, this should be covered by most familiar with content).

Additionally (pulling from a discussion in a private repo), let's make a section: "4. Merge Your Pull Request" which will include information about the merging options.

Basics on _Squash Merge_ from @johnbradley's original Two Repo problem draft:
> - Often there are many messy commits that should be reduced to meaningful commits.
> - One easy way to reduce the messy commits into a single commit is using a `Squash and merge` Pull Request.

Paired with the scope note mentioned in my comment:
> We should be sure to emphasize the fact that once a branch is squash merged into main, it can't continue to be used for development, so they should be scoped. Also, that they should be scoped so the squash merge makes sense and can still be used to tell the story (for instance, one feature change and the associated adjustments to tests).

Start with the linked GitHub PR Guide page and compare its first two sections with GitHub's recent updates, focusing on screenshots, reviewer selection, and branch protections. Add the requested “4. Merge Your Pull Request” section using the provided squash-merge and branch-scoping guidance; the work is done when those updates and the new section are reflected accurately on the guide page.

Written by the indexing model from the issue text.