Add nullable enum guidance: null vs none sentinel values

[OlgaPodo]

Summary

Add a new 'Nullable enums' section to graph/patterns/enums.md with guidance on when to use null vs a none sentinel member for enum properties.

The existing page mentions that 'a nullable enum can very likely be avoided by adding a none member' but does not explain:

• When null is the right choice vs none
• The anti-pattern of combining both
• Decision criteria for API designers

Changes

New #### Nullable enums section added after 'Enum or Booleans', containing:

• null vs none comparison table — clarifies the semantic difference
• Note on unknownFutureValue — reinforces that it is always required and unrelated to nullability
• Prefer none over nullable — recommended pattern with CSDL example
• When nullable is appropriate — three criteria that must all apply
• Anti-pattern: nullable + none — warns against combining both with example

Context

Identified as a guideline gap during Graph API review discussions.