DOC: omit basic_units helper from gallery

PR summary

This PR prevents galleries/examples/units/basic_units.py from being rendered as a standalone gallery page. The file is a helper module used by the units examples, not a visual gallery example, and issue #17479 notes that it should likely be omitted from the gallery listing.

The file is still copied into the generated gallery output via copyfile_regex, so existing examples can continue to import it and users can still download it from links such as :download:\basic_units.py <basic_units.py>``.

Addresses one item from #17479.

AI Disclosure

I used Codex to inspect the issue, read the repository documentation, identify the smallest scoped change, and run local checks. I reviewed the resulting configuration change and verified the regex behavior locally.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

Testing

• git diff --check
• python -m py_compile doc/conf.py
• Local regex check confirming:
  • ../galleries/examples/units/basic_units.py matches ignore_pattern
  • ../galleries/examples/units/basic_units.py matches copyfile_regex
  • ordinary .rst files still match copyfile_regex
  • ordinary example .py files such as bar_unit_demo.py do not match either rule

Full documentation build was not run locally because this environment does not have Matplotlib's documentation dependencies installed, including sphinx_gallery.