Meshery: [Docs] Update GKE and EKS Quick Start Instructions with `mesheryctl system config`

[leecalcote]

Current State:
mesheryctl system config is a new feature to assist in the configuration of the kubeconfig file used by Meshery for interacting with remote, managed Kubernetes clusters operated by Cloud providers.

Desired State:
Given this new feature, let's:

1. Take much of what is on the GKE and EKS pages and move these instructions into a "Manual Configuration" section.
2. Above the "Manual Configuration" section, regurgitate the description of the mesheryctl system config function and its use.
3. Do this on both the GKE and EKS pages.

---

Contributor Resources

• Meshery documentation site
• Meshery documentation source
• Instructions for contributing to documentation

_Originally posted by @leecalcote in https://github.com/layer5io/meshery/pull/1170#issuecomment-648472944_

[leecalcote]

@chandrashritii feel free to grab this one, if you like. 👍

[chandrashritii]

@leecalcote I'll start working on it asap. Thanks for asking me :)

[augustgerro]

Is this feature available for EKS on Darwin?

[chandrashritii]

Is this feature available for EKS on Darwin?

@kushthedude