Add more prominent grid editor docs with image.

[ntouran]

The grid editor is a powerful feature, and not easy to discover or featured in the docs. This adds an intriguing figure (from our fftf isothermal repo) up front so people can see what they're missing and go looking for the feature.

Along the way, I noticed that all png files were ignored across the repo. Since we use png images in our documentation, it's important that we do not blanket ignore such files. I struggled for a while to find out why my png wasn't showing up in the options to stage! If we have build artifacts that are ending up in commits then I suggest we clean those out or be more precise with the ignoring.

Why is the change being made?

This was discussed in https://github.com/terrapower/armi/discussions/1722

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Checklist

• [x] This PR has only one purpose or idea.
• [x] Tests have been added/updated to verify any new/changed code.

• [x] The code style follows good practices.
• [x] The commit message(s) follow good practices.

• [x] The release notes have been updated if necessary.
• [x] The documentation is still up-to-date in the doc folder.
• [x] The dependencies are still up-to-date in pyproject.toml.