documentation cleanup

[DaanHoogland]

note @sudo87 @sureshanaparti @ingox , the double content is not fixed. We also found double content about java upgrades (and unreachable pages)

Originally posted by @DaanHoogland in #11519

The documentation needs a cleanup for double contants and unreachable pages, examples are

https://github.com/apache/cloudstack-documentation/blob/main/source/upgrading/upgrade/_java_version.rst
https://github.com/apache/cloudstack-documentation/blob/main/source/upgrading/upgrade/upgrade_notes.rst

and

https://docs.cloudstack.apache.org/en/latest/adminguide/templates.html#creating-a-linux-template
https://docs.cloudstack.apache.org/en/latest/adminguide/templates/_create_linux.html#creating-a-linux-template

[DaanHoogland]

second example is not an issue (thanks @sudo87 )