Add documentation on pivots

The documentation does not describe pivots and their semantics.

From what I can tell, a) the pivot name maps to one element at a time, b) only the LHS matches or sets a pivot, and the NAC can match on pivots (so the RHS has no interaction with pivots), and c) the pivot will not be set if the NAC matches.

This information should be added to the documentation and extended with examples such as the Petri Net formalism.

No documentation file or entry point is named, so first locate the project's documentation section and read its existing transformation examples. Add an explanation of pivot naming, LHS and NAC interactions, and NAC behavior, with examples such as the Petri Net formalism; done when these semantics are documented clearly.

Written by the indexing model from the issue text.