Documentation: Document the coverage build a bit more

[nico]

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I'm not actually sure if that's the best way to do this. It's based on #23333.

I haven't found a way to run, say, both TestImageWriter and TestImageDecoder and get a report for both runs combined.

There's also a nightly CI bot that's supposedly creating coverage information, but it's been red for years: https://github.com/SerenityOS/serenity/actions/workflows/nightly.yml (We should fix that at some point…)

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This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[stale[bot]]

This pull request has been closed because it has not had recent activity. Feel free to re-open if you wish to still contribute these changes. Thank you for your contributions!