Improve documentation on TreeNode (#10035)

* Improve documentation on `TreeNode`

* Document inspecting, rewriting  APIs. Add chart

* tweak

* Add references to PlanContext and ExprContext

* refine TreeNodeRewriter docs

* Add note about exists

* Update datafusion/common/src/tree_node.rs

Co-authored-by: Jeffrey Vo <[email protected]>

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Co-authored-by: Jeffrey Vo <[email protected]>

Author: alamb