Refine json schema with support for templates

[gberche-orange]

Document how Schema validation can still be applied for goss.yaml files using inline templates, by using render preprocessing

Checklist

• [ ] make test-all (UNIX) passes. CI will also test this
• [ ] unit and/or integration tests are included (if applicable)
• [x] documentation is changed or added (if applicable)

Description of change

Doc update

Would be nice to add some automated tests by using a json schema validator, e.g. one available from https://json-schema.org/implementations.html such as https://github.com/santhosh-tekuri/jsonschema yet its yaml support is pending, see https://github.com/santhosh-tekuri/jsonschema/pull/52

[aelsabbahy]

Thanks for the contribution, noticed a few commits came in after the PR was opened. Let me know when this is ready to merge and I'll merge it.

[gberche-orange]

thanks @aelsabbahy

It would be good for me to add automated tests for the schema to avoid future regressions. I might be able to find time to add this in 2/3 weeks. I may open a new PR if you prefer merging this one sooner.

[aelsabbahy]

I'll merge this now. I'm a bit mixed on automated tests if they'll fail the build, can you check if the v4 branch changes would break this? If not, then it's fine.

[aelsabbahy]

Can't remember if you can comment after I merge or not, I'll keep it open until your next response and then merge it

[gberche-orange]

thanks @aelsabbahy

Might be better for you to merge this branch now, and that I later submit additional tests against v4 branch or after v4 gets merged.

[aelsabbahy]

Thanks for the update and great contribution. Much appreciated!