docs: cqn4sql path expression transformations

[patricebender]

Summary

• Adds a document covering all join-related path expression transformations in cqn4sql
• Covers association-to-join, exists subqueries, expand/inline, infix filters, inner joins in exists, FROM navigation, and UPDATE/DELETE rewrites
• Documents the current limitation around join-relevant paths in infix filters and the planned concept from feat(a2j): enable path expression inside filter #1361

[stewsk]

clarify: what if also another field is accessed via the assoc?

[stewsk]

why "with/without"?

[patricebender]

→ the same association once with a filter and once without a filter produce separate joins

[stewsk]

Suggested change

	Infix filters also apply in `exists` and `FROM` navigation, where they become conditions in the `WHERE EXISTS` subquery.
	Infix filters also can occur in `exists` and `FROM` navigation, where they become conditions in the `WHERE EXISTS` subquery.

[stewsk]

Suggested change

	Both association paths in `FROM` and the `exists` predicate produce `WHERE EXISTS` correlated subqueries. The mechanics are the same — a `FROM` path like `Authors:books` is semantically equivalent to `SELECT from Books where exists author`.
	Association paths both in `FROM` and in an `exists` predicate produce `WHERE EXISTS` correlated subqueries. The mechanics are the same — a `FROM` path like `Authors:books` is semantically equivalent to `SELECT from Books where exists author`.

[stewsk]

Suggested change

	An association step in the `FROM` clause rewrites the query target to the association's target entity and adds a `WHERE EXISTS` subquery correlating back to the source:
	An association step in the `FROM` clause moves the query target to the association's target entity and adds a `WHERE EXISTS` subquery correlating back to the source: