Add a deprecation note in the README in order to archive docker-inbound-agent repository

[lemeurherve]

This PR adds an important note explaining that the content of this repository has been integrated into https://github.com/jenkinsci/docker-agent.

After merging this PR I'll archive the repository.

Follow-up of:

• feat: build both agent and inbound agent docker-agent#570

Ref:

• Build both agent and inbound-agent container images in this repository docker-agent#569 (comment)

Testing done

N/A

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