docs: Add 'Customizing library models for Rust' documentation

Add documentation for customizing library models for Rust using data
extension files. This follows the pattern of existing documentation for
other languages (Java, Python, Ruby, Go, C#, C++, JavaScript).

The documentation covers:
- Rust-specific extensible predicates (sourceModel, sinkModel,
  summaryModel, neutralModel) with their simplified schema
- Canonical path syntax for identifying Rust functions and methods
- Examples using real models from the codebase (sqlx, reqwest,
  std::env, std::path, Iterator::map)
- Access path token reference (Argument, Parameter, ReturnValue,
  Element, Field, Reference, Future)
- Source and sink kind reference
- Threat model integration

Also updates codeql-for-rust.rst to include the new page in the
toctree.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: coadaflorin

docs/codeql/codeql-language-guides/codeql-for-rust.rst

@@ -12,9 +12,12 @@ Experiment and learn how to write effective and efficient queries for CodeQL dat
    basic-query-for-rust-code
    codeql-library-for-rust
    analyzing-data-flow-in-rust
+   customizing-library-models-for-rust
 
 -  :doc:`Basic query for Rust code <basic-query-for-rust-code>`: Learn to write and run a simple CodeQL query.
 
 -  :doc:`CodeQL library for Rust <codeql-library-for-rust>`: When analyzing Rust code, you can make use of the large collection of classes in the CodeQL library for Rust.
 
 -  :doc:`Analyzing data flow in Rust <analyzing-data-flow-in-rust>`: You can use CodeQL to track the flow of data through a Rust program to places where the data is used.
+
+-  :doc:`Customizing library models for Rust <customizing-library-models-for-rust>`: You can model the functions and methods that control data flow in any framework or library by using data extension files.