docs: add C# Math.Atanh() documentation (#7982)

* docs: add C# Math.Atanh() documentation

Added comprehensive documentation for Math.Atanh() method including:
- Description of inverse hyperbolic tangent functionality
- Syntax section with parameter details
- Return value specifications including edge cases
- Practical example with multiple test cases
- Runnable codebyte example

Fixes #7946

* fix: correct formatting and spacing in atanh.md

* minor content fixes

* Update atanh.md

---------

Author: Priyanshjain10

content/c-sharp/concepts/math-functions/terms/atanh/atanh.md

@@ -0,0 +1,82 @@
+---
+Title: 'Atanh()'
+Description: 'Returns the inverse hyperbolic tangent of a number.'
+Subjects:
+  - 'Computer Science'
+  - 'Code Foundations'
+Tags:
+  - 'Arithmetic'
+  - 'Methods'
+  - 'Numbers'
+CatalogContent:
+  - 'learn-c-sharp'
+  - 'paths/computer-science'
+---
+
+The **`Math.Atanh()`** method returns the inverse hyperbolic tangent (hyperbolic arctangent) of a specified number. The inverse hyperbolic tangent is the value whose hyperbolic tangent is the input number.
+
+## Syntax
+
+```pseudo
+Math.Atanh(value);
+```
+
+**Parameters:**
+
+- `value`: A double-precision floating-point number in the range `-1` to `1`, representing the hyperbolic tangent value.
+
+**Return value:**
+
+The `Math.Atanh()` method returns the inverse hyperbolic tangent of the given `value` as a `double`.
+
+It returns:
+
+- `NaN` (Not a Number) if the `value` is less than `-1` or greater than `1`.
+- `NegativeInfinity` if the `value` is exactly `-1`.
+- `PositiveInfinity` if the `value` is exactly `1`.
+
+## Example: Basic Usage of `Math.Atanh()`
+
+The following example demonstrates the basic usage of `Math.Atanh()`:
+
+```cs
+using System;
+
+class Program
+{
+  static void Main()
+  {
+    double value = 0.5;
+    double result = Math.Atanh(value);
+    Console.WriteLine($"Atanh({value}) = {result}");
+  }
+}
+```
+
+This will output:
+
+```shell
+Atanh(0.5) = 0.5493061443340548
+```
+
+## Codebyte Example
+
+In this example, different values are passed to `Math.Atanh()` to observe how the result changes:
+
+```codebyte/csharp
+using System;
+
+class Program
+{
+  static void Main()
+  {
+    double[] testValues = { 0.25, 0.5, 0.75, 0.9, -0.5 };
+
+    foreach (double value in testValues)
+    {
+      double result = Math.Atanh(value);
+      Console.WriteLine($"Atanh({value}) = {result:F4}");
+    }
+  }
+}
+```