Ozone: Support optional documentation link in KSM/SCM webui

In some cases it could be useful to include additional documentation to the SCM/KSM web ui.

This patch includes an optional hook. During the startup scm/ksm web ui do a HTTP HEAD request and if docs/index.html exists, an additional Documentation link will be displayed in the menu header.

Long term we can generate documentation automatically from the source.

Testing:

Do a full build, start scm: no link in the ui.

Add some optional documentation (choose one):

• hadoop-hdfs-project/hadoop-hdfs/src/main/webapps/scm/docs/index.html (before build)
• hadoop-hdfs-project/hadoop-hdfs/target/webapps/scm/docs/index.html (after clean, before build)
• hadoop-dist/target/hadoop-3.1.0-SNAPSHOT/share/hadoop/hdfs/webapps/scm/docs/index.html (after build)

And start scm again. There should be a documentation link in the menu which opens the optional documentation.