Support shared site.contentType.php controller

[distantnative]

Description

Summary of changes

• Allows content representation-specific site controllers, e.g. site.json.php
• Either site.php or the more specific representation site controller get merged into any other controller, but never both

Reasoning

Allows to globally provide data to all controllers/templates that only a specific content type needs.

Additional context

• Please review the unit tests closely if each configuration ends in the results you would expect as well.

Changelog

Enhancement

• Support for content representation specific site controllers, e.g. site.rss.php

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add changes & docs to release notes draft in Notion