docs: pubkey path changes

[jherzstein]

Pending secureblue/secureblue#2200

Changes to contributing due to this file path change for public keys in the above PR. I made sure it doesn't appear anywhere else in the docs.

Also minor correction, we are rebasing to Image Registry, not Image Repository.

Assisted by: GNU grep 3.12

[codacy-production]

Up to standards ✅

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