Enhancement: Exclude firmware zip files from release packages via .gitattributes

## Summary

Modify .gitattributes so that all zip files in the firmware and firmware.labs trees are excluded from future release packages. This will significantly reduce package size now that live fetch and caching from URLs for firmware files has been implemented.

## Requirements
- Update .gitattributes to use export-ignore for all *.zip files in firmware and firmware.labs directories.
- Ensure that release tarballs and zip packages do not include any firmware zip files.
- Document the change in the release process.

## Motivation
With the new live fetch and caching system (see #60), firmware zip files are no longer needed in release packages. Excluding them will reduce download size and improve efficiency for users.

## Acceptance Criteria
- .gitattributes is updated and committed.
- Release packages do not contain any firmware zip files from firmware or firmware.labs.
- Documentation is updated to reflect the new process.

## Reference
- Related to #60