Improve documentation for users on how to setup and configure gardenctl

[dansible]

It would be great to add some more documentation for end users (specifically non-admins) for how they can setup and use gardenctl. Some of the topics:

• General explanation of the Gardener components and how they relate to one another (can be links to existing documentation) -> Garden (Soil) / Project / Namespace / Shoot
• Downloading your OIDC-kubeconfig (or service account kubeconfig)
• Setup ~/.garden/config to point to the kubeconfig
• How to target your garden
• What you have access to in your project (clarify what is a project vs namespace)
• How to target your shoot

[uvwild]

### Me too:

• I can configure .garden/config with the clusters.

But all I can do is gardenctl ls gardens (which shows the entries from the cluster in the config)

Trying to list any other resource gardenctl ls XXXX
returns
Error: the server could not find the requested resource (get XXX.core.gardener.cloud)
(with XXX in projects, seeds,shoots,issues,namespaces)

There is some essential information missing explaining the configuration!

When I try targeting I get
gardenctl -d target --server https://api.ing.garden.otcdemo.gardener.t-systems.net --seed otc-seed --namespace shoot--openedx--openedx
a garden could not be matched for the provided server address: https://serveraddress/

However, curl -l https://serveraddress gives me the API Server

[petersutter]

@uvwild The kubeconfig that you use for the garden config must be the (virtual) garden cluster. It's the cluster that exposed the Project, Shoot, Seed etc. resources