Add documentation for 5 new Bold classes and improve cross-linking

New class documentation:
- TBoldElement - Abstract base class for all Bold elements
- TBoldSystemHandle - Manages the Object Space connection
- TBoldListHandle - OCL list expressions with filtering and sorting
- TBoldExpressionHandle - Single value OCL expressions
- TBoldObjectReference - Single-valued associations

Improvements:
- Add Handle Classes section to mkdocs nav
- Add class hierarchy diagram to OCL Type Operations
- Fix mermaid click links (add href keyword)
- Change relative links from folder/ to file.md format
- Add cross-links between documentation pages

Author: bero

docs/classes/TBoldAttribute.md

@@ -21,7 +21,7 @@ classDiagram
     TBoldAttribute : +SetToNull()
     TBoldAttribute : +AsString
 
-    click TBoldMember href "TBoldMember/" "TBoldMember documentation"
+    click TBoldMember href "TBoldMember.md" "TBoldMember documentation"
 ```
 
 ## Attribute Types

docs/classes/TBoldElement.md

@@ -0,0 +1,181 @@
+# TBoldElement
+
+`TBoldElement` is the abstract base class for all elements in the Bold framework. It provides common functionality for string representation, comparison, assignment, and subscription.
+
+## Class Definition
+
+```pascal
+TBoldElement = class(TBoldSubscribableObject)
+public
+  // String representation
+  property AsString: string;
+  property DisplayName: string;
+  function GetStringRepresentation(Representation: TBoldRepresentation): string;
+  procedure SetStringRepresentation(Representation: TBoldRepresentation; const Value: string);
+
+  // Comparison
+  function CompareTo(BoldElement: TBoldElement): Integer;
+  function IsEqual(BoldElement: TBoldElement): Boolean;
+
+  // Assignment
+  procedure Assign(Source: TBoldElement);
+  function IsEqualAs(CompareType: TBoldCompareType; BoldElement: TBoldElement): Boolean;
+
+  // OCL evaluation
+  function EvaluateExpression(const Expression: string): TBoldElement;
+  function EvaluateExpressionAsNewElement(const Expression: string): TBoldElement;
+
+  // Mutability
+  property Mutable: Boolean;
+end;
+```
+
+## Inheritance
+
+```mermaid
+classDiagram
+    TBoldSubscribableObject <|-- TBoldElement
+    TBoldElement <|-- TBoldDomainElement
+    TBoldElement <|-- TBoldSystem
+
+    TBoldElement : +AsString
+    TBoldElement : +CompareTo()
+    TBoldElement : +EvaluateExpression()
+
+    click TBoldSystem href "TBoldSystem.md" "TBoldSystem documentation"
+```
+
+## Properties
+
+### AsString
+
+Universal string representation:
+
+```pascal
+var
+  Element: TBoldElement;
+begin
+  ShowMessage(Element.AsString);
+end;
+```
+
+### DisplayName
+
+Human-readable name for display purposes:
+
+```pascal
+ShowMessage('Working with: ' + Element.DisplayName);
+```
+
+### Mutable
+
+Indicates whether the element can be modified:
+
+```pascal
+if Element.Mutable then
+  Element.AsString := 'New Value';
+```
+
+## Methods
+
+### CompareTo
+
+Compare two elements:
+
+```pascal
+var
+  Result: Integer;
+begin
+  Result := Element1.CompareTo(Element2);
+  // Result < 0: Element1 < Element2
+  // Result = 0: Element1 = Element2
+  // Result > 0: Element1 > Element2
+end;
+```
+
+### IsEqual
+
+Check equality:
+
+```pascal
+if Element1.IsEqual(Element2) then
+  ShowMessage('Elements are equal');
+```
+
+### Assign
+
+Copy value from another element:
+
+```pascal
+TargetElement.Assign(SourceElement);
+```
+
+### EvaluateExpression
+
+Evaluate an OCL expression with this element as context:
+
+```pascal
+var
+  Customer: TBoldObject;
+  OrderCount: TBoldElement;
+begin
+  OrderCount := Customer.EvaluateExpression('self.orders->size');
+  ShowMessage('Orders: ' + OrderCount.AsString);
+end;
+```
+
+### EvaluateExpressionAsNewElement
+
+Evaluate and return a new element (caller owns it):
+
+```pascal
+var
+  Customer: TBoldObject;
+  ActiveOrders: TBoldObjectList;
+begin
+  ActiveOrders := Customer.EvaluateExpressionAsNewElement(
+    'self.orders->select(status = ''Active'')'
+  ) as TBoldObjectList;
+  try
+    // Use ActiveOrders
+  finally
+    ActiveOrders.Free;
+  end;
+end;
+```
+
+## String Representations
+
+Bold supports multiple string representations for different purposes:
+
+| Representation | Constant | Purpose |
+|----------------|----------|---------|
+| Default | `brDefault` | General display |
+| Short | `brShort` | Compact display |
+| Long | `brLong` | Detailed display |
+
+```pascal
+// Get specific representation
+var
+  ShortName: string;
+begin
+  ShortName := Element.GetStringRepresentation(brShort);
+end;
+```
+
+## Derived Classes
+
+| Class | Description |
+|-------|-------------|
+| [TBoldSystem](TBoldSystem.md) | The Object Space manager |
+| TBoldDomainElement | Base for objects and members |
+| [TBoldObject](TBoldObject.md) | Domain object instances |
+| [TBoldMember](TBoldMember.md) | Attributes and references |
+| [TBoldAttribute](TBoldAttribute.md) | Simple values |
+| [TBoldObjectList](TBoldObjectList.md) | Collections |
+
+## See Also
+
+- [TBoldSystem](TBoldSystem.md) - Object Space manager
+- [TBoldObject](TBoldObject.md) - Domain objects
+- [Subscriptions](../concepts/subscriptions.md) - Change notification