HBASE-29878 Improve the Documentation UI (addendum) - Maven site plugin

[PDavid]

Add back Maven site plugin to fix the warning during javadoc generation.

[WARNING] An issue has occurred with maven-javadoc-plugin:3.4.0:aggregate-no-fork report, skipping LinkageError Receiver class org.apache.maven.plugins.javadoc.AggregatorJavadocNoForkReport does not define or inherit an implementation of the resolved method 'abstract void generate(org.codehaus.doxia.sink.Sink, java.util.Locale)' of interface org.apache.maven.reporting.MavenReport., please report an issue to Maven dev team.
java.lang.AbstractMethodError: Receiver class org.apache.maven.plugins.javadoc.AggregatorJavadocNoForkReport does not define or inherit an implementation of the resolved method 'abstract void generate(org.codehaus.doxia.sink.Sink, java.util.Locale)' of interface org.apache.maven.reporting.MavenReport.
    at org.apache.maven.plugins.site.render.ReportDocumentRenderer.renderDocument (ReportDocumentRenderer.java:239)
    at org.apache.maven.doxia.siterenderer.DefaultSiteRenderer.render (DefaultSiteRenderer.java:349)
    at org.apache.maven.plugins.site.render.SiteMojo.renderLocale (SiteMojo.java:198)
    at org.apache.maven.plugins.site.render.SiteMojo.execute (SiteMojo.java:147)
    at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:126)
    at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute2 (MojoExecutor.java:328)
    at org.apache.maven.lifecycle.internal.MojoExecutor.doExecute (MojoExecutor.java:316)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:212)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:174)
    at org.apache.maven.lifecycle.internal.MojoExecutor.access$000 (MojoExecutor.java:75)
    at org.apache.maven.lifecycle.internal.MojoExecutor$1.run (MojoExecutor.java:162)
    at org.apache.maven.plugin.DefaultMojosExecutionStrategy.execute (DefaultMojosExecutionStrategy.java:39)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:159)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:105)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:73)
    at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:53)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:118)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:261)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:173)
    at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:101)
    at org.apache.maven.cli.MavenCli.execute (MavenCli.java:906)
    at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:283)
    at org.apache.maven.cli.MavenCli.main (MavenCli.java:206)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:77)
    at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke (Method.java:569)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:283)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:226)
    at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:407)
    at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:348)