Clean up Quick Start (Extended) Documentation

[milanvujinovic]

General

• Offer option in the guide to choose between linux or macos variants of guide.

Introduction

• The subtitle (greyed out text) at the top can be made like:
  "Looking for a shorter guide? See the...."
• E2E test suite mentioned but this was never mentioned previously.

Prerequisites

• "Required and optional tools" should use same structure as QuickStart Guide on the required tools. Mention optional only as an aside, not in the same table as required.
• Mentions docker but guide offers podman as well, seems to be only for linux, while Docker is for macos.
• Text from "The project ships a helper script that downloads and verifies ..." until the command to export PATH should be moved at the end of step 1, after checking out the repo.

[zaher-abb]

Re-running "make deploy-infra" example : "WITH_PROMETHEUS=true make deploy-infra" against an existing Forge kind cluster fails while installing Gateway API CRDs. The bootstrap script applies Gateway API "standard-install.yaml" with server-side apply, but the same CRD fields are already managed by Flux's "helm-controller".