Document DataFusion optimizer rules #21771

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Existing docs:

• docs/source/library-user-guide/query-optimizer.md — good high-level overview of the optimizer architecture + how to write custom rules. Does NOT list/describe individual built-in rules.
• datafusion/optimizer/README.md and datafusion/physical-optimizer/README.md — brief crate-level READMEs, mostly pointers to the above doc and blog posts.
• working-with-exprs.md and extending-operators.md — touch on optimizer rules tangentially.

The gap: There is no reference doc that lists and describes what each built-in rule actually does. The codebase has 27 logical optimizer rules (e.g., push_down_filter, eliminate_cross_join,
common_subexpr_eliminate) and 21 physical optimizer rules (e.g., join_selection, enforce_sorting, topk_aggregation), but the only way to learn what each one does is to read the source or use EXPLAIN
VERBOSE.

So query-optimizer.md is good for "how the optimizer works" and "how to extend it," but it's not enough if you want a reference of "what optimizations does DataFusion apply and what does each one do."
That reference doesn't exist yet.

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