Document proper use of link and target

[lukaspie]

It should be clarified in the NeXus documentation how link and target shall be used in NeXus (this should be done together with or directly by NIAC).

How it should work:

From the (NeXus docs)[https://manual.nexusformat.org/design.html#links], it is specified that the target of links in app defs shall be class paths, but this is well-documented and (to my knowledge) not really used properly anywhere.

The idea is: if I have an application definition called NXmy_app_def containing a wavelength in its NXinstrument, which I want to reference somewehere else in the app def in an NXprocess, it should be done like this:

NXmy_app_def(NXobject):
  (NXentry):
    (NXinstrument):
      crystal(NXmonochromator):
        wavelength(NX_NUMBER):
    [...]
    my_process(NXprocess):
     link_field(link):
       target: /NXmy_app_def/ENTRY/INSTRUMENT/crystal/wavelength