Add Support for `css-zero`

[cb341]

Problem

simple_form works well with Bootstrap 5 and Foundation, providing easy form styling. However, it lacks built-in support for the css-zero framework (https://github.com/lazaronixon/css-zero), requiring manual setup.

Solution

Add official css-zero support by:

1. Adding an initializer: A pre-configured file to style simple_form output for css-zero.
2. Creating a generator option: Allow users to run rails generate simple_form:install --css-zero to easily install the setup file.
3. Updating documentation: Provide clear instructions on using simple_form with css-zero.

Why css-zero?

css-zero is a "no build" or class-light CSS approach for Rails, and supporting it would make simple_form more versatile for developers who prefer this framework.

This feature would simplify integrating simple_form into projects using css-zero.