Drop example with descendant path and add wiki page for further discussion

In the spec we have this example, that includes "//*" in a path, which is not currently supported by AQL:

SELECT o
FROM
EHR e CONTAINS
COMPOSITION CONTAINS
OBSERVATION o [openEHR-EHR-OBSERVATION.blood_pressure.v1]
WHERE
exists {"o//*/state[at0007]/items[at0008]"}

On https://discourse.openehr.org/t/aql-new-feature-suggestion-descendant-paths/320/4 most agree on this is not currently supported so I suggested to remove the example from the spec and keep it on the WIKI for further discussion of the "descendant path" feature, use cases (if any), etc.