Modify build publishing

- Add signing plugin
- Add nexus publishing plugin
- Tweak publish to separate artifactory and nexus
  for release vs. snapshot/milestone
- Remove docs publish from root as antora will
  have separate publishing system.
- Modify ci workflow to use publishArtifacts task
  which would then pick either artifactory or nexus.
- Relates #165

.github/workflows/ci.yml

@@ -51,7 +51,7 @@ jobs:
           ARTIFACTORY_USERNAME: ${{ secrets.ARTIFACTORY_USERNAME }}
           ARTIFACTORY_PASSWORD: ${{ secrets.ARTIFACTORY_PASSWORD }}
         run: |
-          ./gradlew build artifactoryPublish \
+          ./gradlew build publishArtifacts \
             -PartifactoryUsername="$ARTIFACTORY_USERNAME" \
             -PartifactoryPassword="$ARTIFACTORY_PASSWORD"
   docs: