RDoc-3828 + RDoc-3829 Conditional filtering

[Danielle9897]

Issue link

https://issues.hibernatingrhinos.com/issue/RDoc-3828/Conditional-expression-support-the-new-when-method.
https://issues.hibernatingrhinos.com/issue/RDoc-3829/Document-in-and-all-in-in-when-conditions

Additional description

• Added a new article: Conditional Filtering, documenting the new RQL when(condition, predicate) syntax.
• @maciejaszyk pls note:
  I used predicate instead of filter in some places to avoid ambiguity:
  • The term “filtering” already describes the overall action of narrowing query results.
  • The term “filter” can be confused with RavenDB exploration-query filters.
  • predicate more precisely describes the second argument of when(condition, predicate)

Type of change

• Content - docs
• Content - cloud
• Content - guides
• Content - start pages/other
• New docs feature (consider updating /templates or readme)
• Bug fix
• Optimization
• Other

Changes in docs URLs

• No changes in docs URLs
• Articles are restructured, URLs will change, mapping is required (update /scripts/redirects.json file, set Documents Moved PR label)

Changes in UX/UI

• No changes in UX/UI
• Changes in UX/UI (include screenshots and description)