Consider casing for keyboard shortcuts

[elstoc]

We use a mix of uppercase and lowercase characters to represent keyboard keys. Our contributing guidelines say we should use lowercase, but I'm actually preferring uppercase now, since it matches what's printed on most keyboards.

We should choose one and fix any inconsistencies.

[Phemisters]

since it matches what's printed on most keyboards.

On the other hand, the darktable preferences -> shortcuts use lower case. 🤷 But I agree that we should make the docs internally consistent.

[github-actions]

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