Reorganize API examples and documentation notebooks

[make-ki]

Closes #1642

Summary

• Move the buildable C API examples from power_grid_model_c_example/ to
  examples/c_api/.
• Rename docs/examples/ to docs/example_notebooks/, including the notebook
  data and license files.
• Update the developer-build CMake path, Sphinx toctree, documentation links,
  and notebook-specific Ruff configuration.
• Preserve the existing C example executable target names, CTest names, and
  behavior.

No C++ wrapper examples or public API changes are included.

Verification checklist

Tested locally on NixOS with GCC 16.2.0 using the gcc-debug CMake preset.

• CMake developer configuration succeeds.
• All three existing C example executables build.
• CTest discovers and passes PGMExample, PGMExampleSerialization, and
  PGMExampleLogging.
• The documented build-script option for running the C example still works.
  The full native workflow was run with:
  bash ./build.sh -p gcc-debug -e
• Sphinx builds with the renamed notebook paths and no missing notebook or
  toctree references.
• Ruff applies the intended notebook-specific exceptions to the moved
  notebooks.
• Repository search finds no stale references to the old directories.
  Remaining power_grid_model_c_example occurrences are preserved target
  names.
• The C API guide and folder-structure page link to the new C example paths.

Additional verification

• All 235 non-example native tests passed.
• All three C example tests passed.
• All pre-commit hooks passed.
• Python tests passed: 1,834 passed, 4 skipped, and 7 expected failures.
• Sphinx generated all 12 notebook pages.
• 179 generated notebook navigation/content links and their target anchors were
  checked with no failures.
• The Sphinx build has no new normalized diagnostics compared with the
  unchanged baseline and resolves two previously broken notebook links.

AI assistance

I used ChatGPT to assist with the reorganization and local verification. I
reviewed the changes and take responsibility for the contribution.