Use spec version as JAX-RS version in docs

Multiple doc links to JAX-RS APIs are broken, as doc generation uses broken reference to JAX-RS API version property.

This patch fixes the issue by referencing valid JAX-RS API spec version property.
In general, the fix makes sense of course, but there is one drawback - it would also affect the version of the jar-rs api implementation in the docs - see the same file (`doc/pom.xml`), line 83 - the same property is reused to replace both `jax-rs-version` and `jax-rs-api-jar-version` entities in the `jersey.ent` file.

Could you please add a new property bellow the changed line with e.g. `<jaxrs.impl.version>` referring to `${jaxrs.api.impl.version}` and than changing the `replace` declaration on the line 83 accordingly (to use the new property)?