Document commands, their usage and documentation consistency

[BobbieGoede]

Besides the --help output, we don't really have a place that actually documents the commands provided by nuxt-module-build, and it seems like the things that are documented are a bit fragmented/inconsistent.

• 1. In the readme package.json example, the deprecated implicit build usage is shown https://github.com/nuxt/module-builder#packagejson.

• 2. Using nuxi build-module will always log a warning to specify the build command explicitly, but passing the build argument actually does not work (not sure if local issue).

• 3. The stub argument is hinted at in the documentation for nuxi build-module (https://nuxt.com/docs/api/commands/build-module), but its function/purpose is not actually described.

• 4. The module starter uses nuxt-module-build prepare which seems to do similar things to the nuxt-module-build build --stub according to its description.

I can fix point 1 and maybe point 2, but for 3 and 4 I'm honestly not sure 😅