Refactor document-list macro for compatibility Jinja2 templates

[precious-onyenaucheya-ons]

What is the context of this PR?

Fixes #3163 Removed .push method which was used for adding file metadata information to a list. This is done to make document list component work with Jinja templates.

How to review this PR

Test the changes in Design system Pull the latest PR (https://github.com/ONSdigital/design-system-python-flask-demo/pull/3) which is in review. Follow the updated readme. Update the macro and test it in Flask.

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