Add extension examples to Section 11

[COUNTERMetrics]

Add examples of extensions recommended for

• syndicated usage
• AI usage metrics (pending completion of consultation)

These extensions should go in section 11.5, reserved elements

[COUNTERMetrics]

Notes: on re-reading the Code, inclusion of syndication guidelines fit more naturally in section 11.1 (platform as namespace), and the AI information in 11.4 (new values for enumerated elements). Section 11.5 is about extensions to the reports; both syndication and AI fit within our existing structures.

[COUNTERMetrics]

Note that AI extensions are now held in a separate issue for R5.1.2

[berndoberknapp]

If I remember correctly the code team has discussed to make additional examples, especially regarding extensions, available on the COUNTER website instead of including them in the Code of Practice? Then this issue could be closed.