resolveContentLink takes the label from the wrong node for a link mixin

[romain-pm]

Part of #749. resolveContentLink derives the label from the wrong node when the link is a mixin.

Its precedence is jcr:title on the content node, then j:linkTitle, then the target's displayable name. That is right when the link is the content (jnt:nodeLink). It is wrong when the link is a mixin on something else: Jahia/jahia.com's jahiacommix:cta sits on a card, a panel or a hero that already has a jcr:title — the heading, not the link label. So the API returns the heading.

This cost the largest single win available on that site: the CTA mixin is 23 call sites, and all of them had to keep getLinkProps plus the discriminator ternary.

• labelProperties?: readonly string[] to state where the label lives.
• And/or labelFrom: "content" | "target".
• Say in the guide which one a mixin-shaped link wants, since that is the common case in a real project.