Add data model overview (gardener#5137)

- Add PNG itself (sorry, no excalidraw, but rfranzke suggested,
  since this figure exists today as PPT, let's add it, which is
  better than having nothing - if it must be extended later, then
  excalidraw can be used to follow the Gardener docs guidelines)
- Mention it where also the depicted resources are mentioned
- "Fix" the heading for (shoot)quotas, because:
  - `quota`/`resourcequotas` refers to K8s resource quotas
  - `squota`/`quotas` refers to Gardener shoot quotas
  That's actually somewhat confusing; the figure therefore
  speaks of shoot quotas and to stay in sync, the heading
  was adapted; ideally, we "fix" that in code with `v1`

docs/concepts/apiserver.md

@@ -26,7 +26,7 @@ The Gardener operator has to either deploy the Gardenlet into the cluster they w
 
 Please see [this](../../example/45-secret-seed-backup.yaml), [this](../../example/50-seed.yaml)(, and optionally [this](../../example/40-secret-seed.yaml)) example manifests.
 
-## `Quota`s
+## Shoot`Quota`s
 
 In order to allow end-users not having their own dedicated infrastructure account to try out Gardener the operator can register an account owned by them that they allow to be used for trial clusters.
 Trial clusters can be put under quota such that they don't consume too many resources (resulting in costs), and so that one user cannot consume all resources on their own.
@@ -67,3 +67,7 @@ Please see [this](../../example/90-shoot.yaml) example manifest and consult the
 ## `(Cluster)OpenIDConnectPreset`s
 
 Please see [this](../usage/openidconnect-presets.md) separate documentation file.
+
+## Overview Data Model
+
+![Gardener Overview Data Model](gardener-data-model-overview.png)