GH-111 - Link to Javadoc from project metadata in reference documentation.

Author: odrotbohm

src/docs/asciidoc/00-preface.adoc

@@ -10,6 +10,7 @@
 * Release repository: Maven central
 * Milestone repository: https://repo.spring.io/milestone
 * Snapshot repository: https://repo.spring.io/snapshot
+* Javadoc: https://docs.spring.io/spring-modulith/docs/{projectVersion}/api
 
 == Using Spring Modulith