Improve the Skaffold installation page

* Remove the tabbing from the [Managed IDE](https://skaffold.dev/docs/install/#managed-ide) section so the Open in Google Cloud Shell link is more discoverable.
* Replace the Open in Google Cloud Shell link with a link to the [new quickstart tutorial](https://ssh.cloud.google.com/cloudshell/editor?show=ide%2Cterminal&cloudshell_git_repo=https://github.com/GoogleContainerTools/skaffold&walkthrough_id=skaffold--skaffold_onboarding&cloudshell_workspace=/examples/buildpacks-node-tutorial&cloudshell_open_in_editor=src/index.js). (Currently, it opens the getting-started directory with no instructions.)
* Add a What’s next section below the installation instructions to direct users to next steps such as tutorials and additional documentation. (e.g., [What’s Next section](https://kubernetes.io/docs/tasks/tools/install-kubectl-macos/#what-s-next) in kubectl installation docs).
* Improve documentation for what is included in the Skaffold Docker images and how to use them to install Skaffold ([relevant GitHub issue](https://github.com/GoogleContainerTools/skaffold/issues/8131)).