Minor: clean up titles and links n extending operators and optimizer pages (#19317)

## Which issue does this PR close?

- Follow on to #19265

## Rationale for this change

While reviewing a nice doc update from @geoffreyclaude I noticed a few
small eyesores in the exsting docs
-
#19265 (review)

<img width="459" height="609" alt="Screenshot 2025-12-14 at 6 52 15 AM"
src="https://github.com/user-attachments/assets/adfea3e0-bd61-4839-9768-815b49998348"
/>


## What changes are included in this PR?

1. Shorten the page titles to make them easier to find 
2. Drive by fix to add links to the opening lines

After this PR:
<img width="461" height="558" alt="Screenshot 2025-12-14 at 6 58 01 AM"
src="https://github.com/user-attachments/assets/005e29de-ebbc-48e8-a070-2585a2fd62f1"
/>


## Are these changes tested?

By ci

## Are there any user-facing changes?

docs only

Author: alamb

docs/source/library-user-guide/extending-operators.md

@@ -17,9 +17,12 @@
   under the License.
 -->
 
-# Extending DataFusion's operators: custom LogicalPlan and Execution Plans
+# Extending Operators
 
-DataFusion supports extension of operators by transforming logical plan and execution plan through customized [optimizer rules](https://docs.rs/datafusion/latest/datafusion/optimizer/trait.OptimizerRule.html). This section will use the µWheel project to illustrate such capabilities.
+DataFusion supports extending operators by transforming [`LogicalPlan`] and [`ExecutionPlan`] through customized [optimizer rules](https://docs.rs/datafusion/latest/datafusion/optimizer/trait.OptimizerRule.html). This section will use the µWheel project to illustrate such capabilities.
+
+[`logicalplan`]: https://docs.rs/datafusion/latest/datafusion/logical_expr/enum.LogicalPlan.html
+[`executionplan`]: https://docs.rs/datafusion/latest/datafusion/physical_plan/trait.ExecutionPlan.html
 
 ## About DataFusion µWheel
 

docs/source/library-user-guide/query-optimizer.md

@@ -17,17 +17,20 @@
   under the License.
 -->
 
-# DataFusion Query Optimizer
+# Query Optimizer
 
 [DataFusion][df] is an extensible query execution framework, written in Rust, that uses Apache Arrow as its in-memory
 format.
 
 DataFusion has modular design, allowing individual crates to be re-used in other projects.
 
 This crate is a submodule of DataFusion that provides a query optimizer for logical plans, and
-contains an extensive set of OptimizerRules that may rewrite the plan and/or its expressions so
+contains an extensive set of [`OptimizerRule`]s and [`PhysicalOptimizerRules`] that may rewrite the plan and/or its expressions so
 they execute more quickly while still computing the same result.
 
+[`optimizerrule`]: https://docs.rs/datafusion/latest/datafusion/optimizer/trait.OptimizerRule.html
+[`physicaloptimizerrules`]: https://docs.rs/datafusion/latest/datafusion/physical_optimizer/trait.PhysicalOptimizerRule.html
+
 ## Running the Optimizer
 
 The following code demonstrates the basic flow of creating the optimizer with a default set of optimization rules