docs: describe the PC0030 setup and reference table exception

[Arthurvdv]

Companion to ALCops/Analyzers#543 (fixes ALCops/Analyzers#531).

PC0030 no longer reports reads on setup and reference tables, because partial records bypass the NST record cache that already serves those few-row tables. This page update:

• adds the exception to the Exception list,
• adds a Setup and reference tables section explaining the cache interaction and the three structural recognition signals (.Setup namespace, single Code PK named Code/Name, the setup-singleton heuristic) plus the AutoIncrement veto,
• states that master data, documents and ledgers are still reported and why,
• links Duilio Tacconi's post in See also.

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