Docs: add how to guide for enabling esm-infra-legacy on Trusty

[lucasmoura]

Why is this needed?

We will soon realize Trusty support for esm-infra-legacy. This PR adds the documentation for enabling this service

Test Steps

Check if the combined page with all Trusty information is properly designed

Checklist

• [ ] I have updated or added any unit tests accordingly
• [ ] I have updated or added any integration tests accordingly
• [ ] Changes here need to be documented, and this was done in:

Does this PR require extra reviews?

• [ ] Yes
• [ ] No

[lucasmoura]

@s-makin @cpaelzer updated