Link to @API's new home in User Guide

This commit changes the link to the @API documentation to point to its
soon-to-be new home (i.e., in the "current" folder which does not yet
exist on the @API Guardian documentation Web site).

Issue: junit-team#436

documentation/src/docs/asciidoc/index.adoc

@@ -24,7 +24,6 @@ ifdef::backend-pdf[:imagesdir: {outdir}/{imagesdir}]
 :junit-platform-engine:             {javadoc-root}/org/junit/platform/engine/package-summary.html[junit-platform-engine]
 :junit-platform-launcher:           {javadoc-root}/org/junit/platform/launcher/package-summary.html[junit-platform-launcher]
 :junit-platform-support-package:    {javadoc-root}/org/junit/platform/commons/support/package-summary.html[org.junit.platform.commons.support]
-:API:                               {javadoc-root}/org/junit/platform/commons/meta/API.html[@API]
 :Launcher:                          {javadoc-root}/org/junit/platform/launcher/Launcher.html[Launcher]
 :ConsoleLauncher:                   {javadoc-root}/org/junit/platform/console/ConsoleLauncher.html[ConsoleLauncher]
 :JUnitPlatform-Runner:              {javadoc-root}/org/junit/platform/runner/JUnitPlatform.html[JUnitPlatform]
@@ -73,6 +72,7 @@ ifdef::backend-pdf[:imagesdir: {outdir}/{imagesdir}]
 :junit5-maven-consumer:             {junit5-samples-repo}/tree/{release-branch}/junit5-maven-consumer[junit5-maven-consumer]
 :MockitoExtension:                  {junit5-samples-repo}/tree/{release-branch}/junit5-mockito-extension/src/main/java/com/example/mockito/MockitoExtension.java[MockitoExtension]
 //
+:API:                               https://apiguardian-team.github.io/apiguardian/docs/current/api/[@API]
 :AssertJ:                           http://joel-costigliola.github.io/assertj/[AssertJ]
 :Gitter:                            https://gitter.im/junit-team/junit5[Gitter]
 :Hamcrest:                          http://hamcrest.org/JavaHamcrest/[Hamcrest]