[DOC] Testing Framework documentation page is marked "under construction" and incomplete

[direkkakkar319-ops]

Describe the documentation issue

The Testing Framework page in the skbase documentation is explicitly
self-marked as "under construction":

"This page is under construction. We plan to add more tests and increased
documentation on the testing framework in an upcoming release."

Location: https://skbase.readthedocs.io/en/latest/contribute/development/developer_guide/testing_framework.html

The page currently only contains three bullet points about the tiered test
approach (package level, module level, low level) with no actual guidance,
examples, or explanation of how to use the testing framework.

This is a significant gap because skbase.testing is one of skbase's four
core submodules (alongside skbase.base, skbase.lookup, and
skbase.validate). A new contributor or downstream package developer
visiting this page gets almost no useful information and is forced to
read raw source code instead.

Suggest a concrete fix/improvement

Complete the testing_framework.rst page to include:

• A clear explanation of each testing tier with concrete examples
• How to run the full test suite and a single test file locally
• How to use skbase.testing tools — BaseFixtureGenerator, QuickTester,
  TestAllObjects — with a worked code example
• How to use create_test_instance and create_test_instances_and_names
  in tests with a worked code example
• How to write and register new tests for a BaseObject subclass

File to edit: docs/source/contribute/development/developer_guide/testing_framework.rst