Add script to install required packages from documentation

[shaun-nx]

Idea

A lot of the documentation, such as the Get Started and Install NGINX Gateway Fabric with Helm ask that users have tools such as Docker, Helm and Kubectl installed before proceeding.

In the example of the Get Started page, I feel it would be helpful to provide a script that installs these dependencies for the user.

The example I made of this script will check if the user has the dependencies installed and display the version installed upon completion.

Right now, this version of the script uses brew as the package manager. This could be expanded to support other package managers.

Open to any and all feedback.

I uploaded some recordings to asciinema to show how the script works toady.

Install packages that are not yet present

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Install packages that are already present