Document `ignore` trigger

[pirat89]

Documents changes in following PRs:

• https://github.com/packit/packit/pull/2234

I keep the list of supported triggers for vm_image_build and propose_downstream untouched as it's not clear to me yet whether the ignore trigger will be available everywhere or not. Let me know in case of wanted changes.

Also, feel free to do any changes in this PR - no need to ask for an approval.

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[lbarcziova]

regate

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