Document the Environment typed readers #153

Summary

Document the typed readers of the Environment package once they ship (quantum-php/framework#614, #615, #616).

Why

Values from .env are text, and the docs do not say so. Anyone writing a config file needs to know that env('DEBUG') is the string 'false', and that env_bool() and env_int() exist.

Pages

• v3.0/packages/environment/contracts.md: the contract of getBool() and getInt(), including the rules and the exception for text that is not a whole number.
• v3.0/packages/environment/helpers.md: env_bool() and env_int(), next to env().
• v3.0/packages/environment/usage.md: an example of a config file that converts at the source, using only names that exist in the code.
• v3.0/packages/environment/architecture.md and overview.md: the new EnvironmentInterface and the traits, when the refactor has shipped.
• v3.0/upgrade-guide.md: a short note that nothing breaks and that new config files should use the helpers.

Acceptance criteria

• every name in the examples exists in the released code
• each fact is stated once at its home page and linked elsewhere
• internal links and anchors resolve
• the pull request goes into staging and is published with the next documentation release