Add gradle build files

- First set of changes for introduction of a gradle build.
- Relates #470

spring-shell-docs/spring-shell-docs.gradle

@@ -0,0 +1,15 @@
+plugins {
+	id 'org.springframework.shell.docs'
+}
+
+description = 'Spring Shell Documentation'
+
+dependencies {
+	management platform(project(":spring-shell-management"))
+	implementation project(':spring-shell-starter')
+	testImplementation 'org.springframework.boot:spring-boot-starter-test'
+}
+
+asciidoctorj {
+	version = '2.5.4'
+}