Overhaul gradle build

- Focus of this commit is to have modern gradle build.
- Migrate most of a plugin configurations from dsl
  into buildSrc.
- This fixes issues with existing docs build.
- Allows to sign files so that we have that part
  done for central in a build.
- We can skip publishing samples.
- We're able to share similar logic for modules
  which are meant for publish or just being samples.
- It's easier to upgrade gradle versions without
  getting various build issues.
- Relates #1143

docs/docs.gradle

@@ -0,0 +1,9 @@
+plugins {
+	id 'org.springframework.statemachine.docs'
+}
+
+description = 'Spring Statemachine Documentation'
+
+asciidoctorj {
+	version = '2.5.4'
+}