Refresh "Getting Started" documentation

[JeanChristopheMorinPerso]

The getting started documentation at https://rez.readthedocs.io/en/stable/getting_started.html is getting old. It's still using rez-bind --quickstart which we would like to deprecate, the package build example uses the cmake integration, etc.

A good getting started page would instead:

• Explain how to create the os, platform and arch packages, with a note to mention that this is platform specific, etc.
• Explain how to bootstrap a python package. This will be hard, but we can do out best to point at resources, etc.
• Show a minimal package + build it.
• Show how to rez-env
• Clearly guide our users on what the next steps are (configuring packages_path, how to install packages from PyPI, etc).

I am probably missing, but this would be a good starting point.

Requirements:

• https://www.bestpractices.dev/en/projects/8389?criteria_level=1#documentation_quick_start