Handbook: Create image/gif tips section and SSoT chapter for ImageMagick, document image drop shadow

Why is this change being made?

While adding a screenshot to the release post in !106494 (merged) I wanted to add a drop shadow to the image. I have found an ImageMagick convert CLI command which works reliably but was not documented in our handbook yet in https://about.gitlab.com/handbook/tools-and-tips/ where we refer to ImageMagick already.

Since this change adds a second option next to Resizing images and Creating gifs, I have created a new main section for image and gif tips.

In addition to the changes, the ImageMagick installation is now a SSoT with macOS/Linux installation instructions, and linked from all chapters, including the product handbook which is the SSoT for making gifs.

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