Allow configuration for internal docker registry volume

[kwiatekus]

Description

Expose internal docker registry storage size in the serverless CR:

  spec:
    dockerRegistry:
      enableInternal: true
      size: 20Gi

• when edited in the DockerRegistry CR the operator should reconfigure the PVC
• operator should execute a rollout restart of the docker registry workload in order to allow volume expansion
• provide documentation about this configurability option
• update outdated documentation
• when actual PVC size is different from desired (default=20Gi) than DockerRegistry CR should result in warning condition
• actual used size is shown in the status

Reasons

At the moment it it a fixed value of 20 Gi and for some users who use serverless extensively it is not enough.
Additionally, if we expose it for other usecases as well (kyma-project/kyma#18555) than the need to expand it would become even more urgent.

[kwiatekus]

Depends on #19