add additional help context to azd ai

Bring over the --help improvements from https://github.com/Azure/azure-dev/pull/8376.

- Consistent formatting between help commands. Some have underlined headings, some don't. None of the azd ai have color formatting for commands and flags like azd --help does
- Include the Environments & Environment Variables section that explains what environment variables are used to power azd ai experiences

This helps users and helps agents