Add Interlink json

[jaapio]

Other projects using doctrine can be able to link to the interlink index if doctrine would provide this in there documentation. In phpDocumentor guides we implemented the format that is defined by sphinx to create links between documentation sets. The main advantage over hard links to specific pages is that you can link to anchors set by the writers of the external documentation. This will work even if the section is moved to another page and will just require rerendering when links are broken. The interlink format can be used by doctrine documentation itself as well to link between projects. No more hard links between dbal and ORM or ORM and collections.

To support this for doctrine we just need to enable that theme in the documentation rendering, which can be done next to the doctrine theme. It doesn't require any extra work on the documentation itself.

Examples can be found here: https://github.com/phpDocumentor/guides/tree/main/tests/Integration/tests/interlink

I'm willing to create a PR for this feature.

[greg0ire]

I think it could indeed be useful, we do have a few of those links.