chore: document use of custom storage classes

[Steffen911]

Important

Adds documentation for configuring custom storage classes in the Langfuse Helm chart.

• Documentation:
  • Adds section on configuring custom storage classes in README.md.
  • Describes global and component-specific storage class configurations for postgresql, redis, clickhouse, and s3.
  • Clarifies default behavior when no storage class is specified.

^{This description was created by for c7fa4b2. It will automatically update as commits are pushed.}