Reference Hamcrest Javadoc via javadoc.io

Due to an SSL/TLS issue with hamcrest.org, the Dokka task fails with the
following.

> Failed to download package-list from https://hamcrest.org/JavaHamcrest/javadoc/2.1/package-list,
> this might suggest that remote resource is not available, module is
> empty or dokka output got corrupted

See: https://github.com/hamcrest/JavaHamcrest/issues/280

As a workaround, this commit switches to javadoc.io to reference the
Hamcrest Javadoc APIs.

gradle/docs-dokka.gradle

@@ -17,7 +17,7 @@ tasks.findByName("dokkaHtmlPartial")?.configure {
 				url.set(new URL("https://kotlin.github.io/kotlinx.coroutines/"))
 			}
 			externalDocumentationLink {
-				url.set(new URL("https://hamcrest.org/JavaHamcrest/javadoc/2.1/"))
+				url.set(new URL("https://javadoc.io/doc/org.hamcrest/hamcrest/2.1/"))
 			}
 			externalDocumentationLink {
 				url.set(new URL("https://javadoc.io/doc/javax.servlet/javax.servlet-api/latest/"))