Allow adding private registries on the organization level

[pat-s]

Clear and concise description of the problem

Currently private registries can only be added on a repository level via woodpecker-cli registry add.

Suggested solution

Add a --organization flag, similar as in woodpecker-cli secret add --organization.

Alternative

No response

Additional context

No response

Validations

• [X] Read the Contributing Guidelines.
• [X] Read the docs.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.