IBX-113: Fixed content relation configuration

[ciastektk]

https://issues.ibexa.co/browse/IBX-113

This PR provides changed semantic configuration to set relations between contents. Previously field configuration was mistaken added under "api" key but now was moved as a separate node.

In doc there is also missing information about how to export content with content relation field.

To use already implemented RelationMapper (https://github.com/ezsystems/ezrecommendation-client/blob/master/src/lib/Mapper/RelationMapper.php) and export content relation field we need to describe relations between contents e.g.:

(content_type_from.field: content_type_to.field)

ezrecommendation:
    system:
        default:
            field:
                relations:
                    article.image: image.image

For version 4.X namespace "ezrecommendation" should be dropped and configuration for recommendation client should be move to internal ibexa platform namespace.

[Steveb-p]

The validation function is, well, invalid 😛

Some examples are not consistent, but those are not part of the PR (just indentation changes), so I don't mind those.

[Steveb-p]

LGTM. Ideally we would have a test for that configuration so that it won't happen again, but I don't think the bundle is easily compatible with KernelTestCase (i.e. bundle might rely on other bundles that require extended configuration).

👍

[mikolajkaraczyn]

QA Approved on ezPersonalization master