Super scaffold generator updates

[jagthedrummer]

We added some new generators in #602 and I just wanted to capture some ideas for additional things we may want/need to do.

• Write some tests for the generators
• Update bin/super-scaffold to use the generators under the hood (Update tests with rails generate super_scaffold and add deprecation warning bullet_train#1203)
• Update bin/super-scaffold to output a message saying that it's deprecated (Update tests with rails generate super_scaffold and add deprecation warning bullet_train#1203)
• Update docs to use rails g ... instead of bin/super-scaffold ... (Update documentation for rails generate super_scaffold #699)
• Remove usage/documentation from the main scaffolding script
• Move code generation/manipulation stuff directly into the generators

[gazayas]

I'm curious if we can skip the first task ("Write some tests for the generators") since we use the generators in the Super Scaffolding system test setup script?

https://github.com/bullet-train-co/bullet_train/blob/4d8132887f2fc47ab49a960c3f6dc9492ea254d7/test/bin/setup-super-scaffolding-system-test#L6-L8