Replace the old atmosphere-only package guides with comprehensive
coverage of both npm and Atmosphere ecosystems:
- Add using-npm-packages guide with Meteor-specific npm patterns
- Add writing-npm-packages guide with publishing best practices
- Expand using-atmosphere-packages with atmosphere-vs-npm comparison
- Expand writing-atmosphere-packages with devOnly and Npm.devDepends()
support introduced in Meteor 3.4
- Remove superseded old atmosphere guides (#2 and #3)