[DOC] Complete testing framework documentation page

[direkkakkar319-ops]

Reference Issues/PRs

Fixes #524

What does this implement/fix? Explain your changes.

The testing framework documentation page was explicitly marked as
"under construction" with only 3 bullet points and no real content.

The page docs/source/contribute/development/developer_guide/testing_framework.rst
has been completely rewritten (~290 lines) covering:

• Running tests locally — full suite, single file, single test function
• Test module architecture — 3-tier approach (package, module, low-level)
  with naming conventions
• skbase.testing module — detailed docs for BaseFixtureGenerator,
  QuickTester, and TestAllObjects
• Worked examples for get_test_params, create_test_instance,
  create_test_instances_and_names, and run_tests
• How to write and register new tests for a BaseObject subclass
• Testing utilities — brief mention of internal helpers

No index changes were needed — testing_framework.rst was already
linked in developer_guide.rst (toctree line 16, card button line 101).

doc8 linter passes with 0 errors.

Does your contribution introduce a new dependency? If yes, which one?

No new dependencies introduced. This is a documentation-only change.

What should a reviewer concentrate their feedback on?

• Accuracy of the worked code examples — do they correctly reflect
  how BaseFixtureGenerator, QuickTester and TestAllObjects work?
• Completeness — are there any important testing patterns that are
  missing and should be added?
• Language clarity — is the page beginner-friendly enough for new
  contributors?

Any other comments?

Since skbase.testing is one of skbase's four core submodules, having
no real documentation for it was one of the most impactful gaps for new
contributors. This PR addresses that fully.

PR checklist

For all contributions

• I've reviewed the project documentation on contributing
• I've added myself to the list of contributors.
• The PR title starts with either [ENH], [CI/CD], [MNT], [DOC], or [BUG] indicating whether
  the PR topic is related to enhancement, CI/CD, maintenance, documentation, or a bug.

For code contributions

• Unit tests have been added covering code functionality
• Appropriate docstrings have been added (see documentation standards)
• New public functionality has been added to the API Reference

[direkkakkar319-ops]

Hey @fkiraly just raised this PR to fix Issue #524 — the testing framework
docs page that was marked "under construction". Completely rewrote it
with real examples and full coverage of skbase.testing. Would appreciate
a review when you get a chance. Thanks!

[fkiraly]

This is great, but please do not use AI indiscriminately.

• do not change unrelated parts of the documentation, e.g., minuscule reformats or random deletions
• it would be great if you distinguished the tests for scikit-base itself, and the test framework that can be used for third party libraries.

[direkkakkar319-ops]

hi, @fkiraly , added distinguished the tests for scikit-base and removed the unrelated parts .
would appreciate your review