docs: add example how to set a var from file include

[MentalGear]

Faced this issue for days, until I found this in some outside code example. I think it should be clearer that outside files can be included and transformed using a var, especially since the expected idiomatic approach ({% set example_output = include 'example.json' % }) does not work.

Summary

Proposed change:

Add example to the docs:

{% set fileAsVar %}
     {% include 'example.json' %}
{% endset %}

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).