Add PLC modules how-to: I/O tables and linking a wire to a point
element_linking.md already documented the plcMasterData XML shape
(§5) but not the workflow: authoring the I/O table in the Element
Editor, setting a slave's state to 'Esclave PLC', and using
PlcLinkWidget (the 'Automate (PLC)' properties tab) to link a slave
to a specific row.
Includes one thing not obvious from either the UI or a skim of the
code: a PLC master's max_slaves is forced to -1 and
MasterElement::isFull() has no PLC-specific branch, so a PLC master
with every I/O row already linked still shows up in the linking tree
(rows struck through) rather than disappearing the way an ordinary
maxed-out master does. Verified directly in masterelement.cpp before
writing it down.
Linked both ways with element_linking.md and added to _Sidebar.md
under Guides.
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