Octane - Update component generators explanations

[jenweber]

In the CLI guides, we have a section on generating components, https://cli.emberjs.com/release/basic-use/cli-commands/

In Octane, ember g component gets you just the test and template. Flags are needed to generate a glimmer-based or classic component, and we should cover them in this guide. The CLI guides are not versioned, so we need to make sure that we explain both Octane and non-Octane behavior.

This work should target master and will be merged in at 3.14 release.

[gokatz]

I'd love to pick this up.