docs: Add C++ unordered_set::count() term entry (#8027)

content/cpp/concepts/unordered-set/terms/count/count.md

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+---
+Title: 'count()'
+Description: 'Returns the number of elements with a specific key in an unordered_set.'
+Subjects:
+  - 'Code Foundations'
+  - 'Computer Science'
+Tags:
+  - 'Methods'
+  - 'Sets'
+  - 'STL'
+CatalogContent:
+  - 'learn-c-plus-plus'
+  - 'paths/computer-science'
+---
+
+The **`count()`** method checks whether a given key exists in a `std::unordered_set`. Since this container stores only unique elements, `count()` will always return either:
+
+1.  `1`: If the element is found in the set.
+2.  `0`: If the element is not found in the set.
+
+This method is commonly used as a fast, O(1) average time complexity way to check for element existence.
+
+## Syntax
+
+```pseudo
+unordered_set_name.count(key);
+```
+
+**Parameters:**
+
+- `key` (const Key&): The value of the element to search for. Must be of the same type as the elements stored in the `unordered_set`.
+
+**Return value:**
+
+Returns an integer. `1` if the element exists, `0` otherwise.
+
+## Example
+
+This example demonstrates using `count()` to check for the presence of elements within a set of strings:
+
+```cpp
+#include <iostream>
+#include <string>
+#include <unordered_set>
+
+int main() {
+  std::unordered_set<std::string> inventory = {
+    "Sword",
+    "Shield",
+    "Potion"
+  };
+
+  std::cout << "Inventory contains:\n";
+  for (const auto& item : inventory) {
+    std::cout << "- " << item << "\n";
+  }
+
+  // 1. Check for an existing element
+  if (inventory.count("Sword")) {
+    std::cout << "\n'Sword' is present (Count: " << inventory.count("Sword") << ").\n";
+  }
+
+  // 2. Check for a missing element
+  if (inventory.count("Axe") == 0) {
+    std::cout << "'Axe' is not present (Count: " << inventory.count("Axe") << ").\n";
+  }
+
+  return 0;
+}
+```
+
+The output of the code is:
+
+```shell
+Inventory contains:
+- Potion
+- Shield
+- Sword
+
+'Sword' is present (Count: 1).
+'Axe' is not present (Count: 0).
+```
+
+## Codebyte Example
+
+Use the Codebyte below to check for the presence of an item in a set of integers:
+
+```codebyte/cpp
+#include <iostream>
+#include <unordered_set>
+
+int main() {
+  std::unordered_set<int> unique_ids = {101, 205, 330};
+
+  int search_key = 205;
+  int missing_key = 400;
+
+  // Check the count for the element 205
+  std::cout << "Count for " << search_key << ": "
+            << unique_ids.count(search_key) << "\n";
+
+  // Check the count for the element 400
+  std::cout << "Count for " << missing_key << ": "
+            << unique_ids.count(missing_key) << "\n";
+
+  return 0;
+}
+```