remove SigMFGenerator from repository

[Teque5]

This was a good idea but after some consideration
it was out of scope for the work here.

[Copilot]

🟢 Approval recommended

The removal is consistent across implementation, exports, tests, and documentation, with no identified blocking issues.

No open issues 🎉

What changed in this PR

Removes synthetic signal generation from sigmf-python because it is outside the project’s scope.

Changes:

• Removes SigMFGenerator, its exception, exports, tests, and documentation.
• Updates the file-writing example and archive-test comment to reference sigmf.fromarray.
• Bumps the package version to 1.15.0.

File	Description
tests/​test_siggen.py	Deletes generator tests.
tests/​test_archive.py	Updates a comment to reference fromarray.
sigmf/​sigmffile.py	Replaces the generator-based documentation example.
sigmf/​siggen.py	Deletes the generator implementation.
sigmf/​error.py	Removes the generator exception.
sigmf/​__init__.py	Removes generator exports and bumps the version.
docs/​source/​siggen.rst	Deletes the generator guide.
docs/​source/​index.rst	Removes the generator navigation entry.
docs/​source/​api.rst	Removes the generator API entry.

_{Review effort: Balanced}