Make it a reusable workflow, implement it and document it

[javirln]

The goal of the task is to provide a reusable workflow extract from the one introduced here: #822

With the following:

• The reusable workflow needs to automatically onboard workflow if they don't exist by using the already existing reusable workflow: https://github.com/chainloop-dev/labs/blob/main/.github/workflows/chainloop_onboard.yml
• It needs to expose the same optional parameters as the previous workflow.
• It needs to accept a comma separated list of values as additional_materials that on top of the default materials being attested on the GitHub release, are added to the attestation.
• Only the CHAINLOOP_TOKEN is a required input to the workflow.

Why string list of values? Because there is no array type on GitHub workflows. A comma separated values will be used.

Document all the process in docs.chainloop.dev guides section.

[migmartri]

Makes sense to me. I'd make explicit that the only required parameter will be the CHAINLOOP_TOKEN