Includes source links in dokka docs (#646)

Now they're clickable directly from the dokka docs (see the "source"
link in the screenshot)

<img width="1512" alt="Screenshot 2023-11-21 at 12 45 21 PM"
src="https://github.com/slackhq/slack-gradle-plugin/assets/1361086/b79127b5-4ac2-4421-b302-78d86bece2cd">


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build.gradle.kts

@@ -283,6 +283,17 @@ subprojects {
               .toURL()
           )
         }
+        sourceLink {
+          localDirectory.set(layout.projectDirectory.dir("src").asFile)
+          val relPath = rootProject.projectDir.toPath().relativize(projectDir.toPath())
+          remoteUrl.set(
+            providers.gradleProperty("POM_SCM_URL")
+              .map { scmUrl ->
+                URI("$scmUrl/tree/main/$relPath/src").toURL()
+              }
+          )
+          remoteLineSuffix.set("#L")
+        }
       }
     }