Support Antora versions for project documentation links

[sjohnr]

The projects API should support the same options as Contentful. The new antora boolean is needed for the create release endpoint. See spring-io/spring-website#262.

Context:

Antora uses the x.y and x.y-SNAPSHOT patterns for version URLs instead of x.y.z and x.y.z-SNAPSHOT. Therefore, the display version (x.y.z) and the version used in URLs (x.y) are different. This allows the spring.io website to display the latest patch version available for each supported generation, while keeping the URL consistent between patch versions.