docs: update copy promotion step behaviour and configuration

[robinlioret]

Description

This PR updates the "copy" promotion step documentation:

• adds the "ignore" configuration option reference
• adds more details about the behaviour of the step

Checklist

Eligibility

• Linked to an existing issue with no blocking labels (kind/proposal, needs discussion, needs research, maintainer only, area/security, size/large, size/x-large, size/xx-large).
• Changes documentation only.
• Changes ten lines or fewer.

Quality

• Adds or updates corresponding tests.
• Adds or updates corresponding documentation.

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• By a human without AI assistance.
• By a human with AI assistance. A human has reviewed every line prior to opening the PR.
• By an AI with human supervision. A human has reviewed every line prior to opening the PR.
• Entirely by an AI. No human has reviewed this prior to opening the PR.

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[fuskovic]

.gitdirectories are always ignored, regardless of theignore` patterns.

This isn't true. We even have a test for this that allows overriding default ignore rules that asserts a file in .git is included by using "!.git/"

https://github.com/akuity/kargo/blob/main/pkg/promotion/runner/builtin/file_copier_test.go#L302

So I would change that portion to something like:

".git is ignored by default but can be overridden by a user-supplied negation pattern"