Add instructions on how to use self-hosted agents in Azure DevOps

[mariamedp]

The template currently uses Microsoft-hosted agents to run pipelines in Azure DevOps, which is the simplest way to run the jobs and very useful to set up a quick MLOps demo/showcase. However, our customer usually has some specific requirements, for example, security configuration, dependent software needed, etc.; and it can be easier with self-hosted as it gives us more control. Also, the private agent has performance advantages, for example, the ability to run incremental builds, start a job faster, etc.

The documentation needs to have instructions on how to set up a self-hosted agent and how to modify the template pipelines to use it.