Additional section 'OpenAIRE CERIF metadata' to linking-datasets.rst

[ACz-UniBi]

As discussed in the last PID2INST meeting on Wed., Aug. 2nd 2023, here is my first contribution with an additional section on how the OpenAIRE Guidelines for CRIS Managers handle "Instruments" which generate dataset(s).

Please let me know your opinion and suggestions.

[RKrahl]

Dear @ACz-UniBi, sorry that I'm very late with my comments, even after Lou already merged your PR! But:

• Your example shows how to link a dataset with an instrument using some "Institution assigned unique equipment identifier" and not an instrument PID. Given that we are all about instrument PIDs, this is somewhat unfortunate. Could you provide an example using a PID instead?
• The formatting of the XML snippets in your example looks screwed, in particular the indentation.