combine CLI headless documentation with CLI command reference

[josefaidt]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

n/a

Describe the solution you'd like
A clear and concise description of what you want to happen.

Move most of the command reference docs out of https://docs.amplify.aws/cli/usage/headless/ in favor of command reference pages. Migrate this doc to generic CI/CD usage guides:

• how do I use Amplify CLI in my own CI/CD pipeline?
• best practices when working with Amplify CLI headless mode/CI environments

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.