Add javadoc links in build.gradle and update Github action workflow

Introduced a function in build.gradle.kts to add external javadoc links. This increases the quality and completeness of generated javadocs by providing related external references. Also, the Github Actions workflow has been updated to use an 'allJavadocs' task for generating a combined javadocs for all the projects, facilitating easier maintenance and review.

.github/workflows/javadocs.yml

@@ -19,7 +19,7 @@ jobs:
       - name: Build Javadocs
         run: |
           echo "Building javadocs with gradle"
-          ./gradlew clean javadoc
+          ./gradlew clean allJavadocs
           echo "Moving to javadocs directory:"
           cd build/docs/javadoc
           echo "Creating .nojekyll to have Github pages deploy html as is:"

build.gradle.kts

@@ -137,3 +137,27 @@ dependencies {
     api(project(":threading"))
     api(project(":updater"))
 }
+
+fun applyJavaDocOptions(options: MinimalJavadocOptions) {
+    val javaDocOptions = options as StandardJavadocDocletOptions
+    javaDocOptions.links(
+            "https://javadoc.io/doc/com.google.code.findbugs/jsr305/latest/",
+            "https://javadoc.io/doc/org.jetbrains/annotations/latest/",
+            "https://docs.oracle.com/en/java/javase/${java.toolchain.languageVersion.get().asInt()}/docs/api/",
+            "https://javadoc.io/doc/com.fasterxml.jackson.core/jackson-core/latest/",
+            "https://javadoc.io/doc/com.fasterxml.jackson.core/jackson-annotations/latest",
+            "https://javadoc.io/doc/com.fasterxml.jackson.core/jackson-databind/latest",
+            "https://jd.papermc.io/paper/1.19/"
+    )
+}
+
+tasks {
+    register<Javadoc>("allJavadocs") {
+        applyJavaDocOptions(options)
+
+        destinationDir = file("${buildDir}/docs/javadoc")
+        val projects = project.rootProject.allprojects.filter { p -> publicProjects.contains(p.name) }
+        setSource(projects.map { p -> p.sourceSets.main.get().allJava })
+        classpath = files(projects.map { p -> p.sourceSets.main.get().compileClasspath })
+    }
+}