Missing signing metadata doc for apt packages

Describe the bug
The signing metadata doc update from https://github.com/pulp/pulp-operator/pull/1351/files#diff-9752de667409027e79558377cbbc4166e632bc9adf0ad6e9bc44bffc7b7506b6 were published on the wrong place, not getting available in https://pulpproject.org.

Expected behavior
We should copy/move it to https://github.com/pulp/pulp-operator/tree/main/staging_docs.

Additional context
In addition to moving it to the right place, we should also do the following changes:

• fix the sample script for signing apt packages (it worked with the pulp_deb doc sample)
• provide more details and a better explanation on how to configure the signing service (for example, explaining the public key used in the pulpcore-manager command and/or what are the automated steps from https://pulpproject.org/pulp_deb/docs/user/guides/signing_service/#example-signing-script that the operator does and users don't need to do manually)

Start by comparing the documentation changes referenced in pulp-operator pull request 1351 with the staging_docs directory, then review the pulp_deb signing service example and the pulpcore-manager command. Done means the signing metadata documentation is available from pulpproject.org, the sample script works, and the signing-service configuration and operator-managed steps are explained.