docs(cypher): document the openCypher read subset accurately

The Supported-Cypher section was stale (listed WITH/COLLECT/OPTIONAL MATCH as
unsupported — all now work). Rewrite it to list the actual clauses, patterns,
WHERE operators (incl. =~ and EXISTS{}), aggregates and scalar functions, and
note that anything outside the subset now fails with a clear 'unsupported'
error instead of returning empty.