Document the type of keystore expected

For the Maven goal `nbm:nbm` we need to document the [type of keystore expected](https://bits.netbeans.org/mavenutilities/nbm-maven-plugin/nbm-maven-plugin/nbm-mojo.html#keystore).

I'm actually not sure what the answer, but I would assume it would be JKS. The reason why it is important to document this is because the JDK is head-over-heals on its way away from JKS in favor of PKCS#12. Or at least, that is my impression.

Today JKS is a legacy format which there is no need for anymore in the JDK. As of Java 9, PKCS#12 is the default keystore format. From what I understand, the JDK's support for the PKCS#12 caught up with that of JKS in Java 8 meaning from this version onwards the JKS format no longer had any edge over PKCS#12 format, feature wise. Hence, JKS is today redundant.

For now, this ticket is solely about documenting what it expected by the `nbm-maven-plugin`, not to implement support for something else than JKS.

Start with the `nbm:nbm` Maven goal and its `keystore` documentation at the linked nbm-maven-plugin reference. Verify which keystore type the plugin expects, then update the relevant documentation so the format is stated explicitly; the issue is done when users can determine whether JKS or PKCS#12 is required.

Written by the indexing model from the issue text.