add description and examples for const keyword (#107)

This PR addresses #87.

content/2020-12/validation/const.markdown

@@ -14,3 +14,63 @@ related:
   - vocabulary: validation
     keyword: type
 ---
+
+The `const` keyword in restricts an instance to a specific value. Its usage is functionally similar to an `enum` with a single value. Instances validate successfully only if their property value deeply matches the specified constant.
+
+* Applies to various JSON data types, including numbers, strings, booleans, objects, and arrays.
+* Takes precedence over other validation keywords like `type` and `enum`.
+
+{{<alert>}}
+_**Note:** It is best practice to avoid using the `type` keyword or any other validation keyword with `const`, as `const` takes precedence over them. Therefore, it is better not to use them together._
+{{</alert>}}
+
+## Examples
+
+{{<schema `Schema with a specific string value`>}}
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "const": "hello"
+}
+{{</schema>}}
+
+{{<instance-pass `An instance matching the const value is valid`>}}
+"hello"
+{{</instance-pass>}}
+
+{{<instance-fail `An instance not matching the const value is invalid.`>}}
+"world"
+{{</instance-fail>}}
+
+{{<schema `Schema with a specific number value`>}}
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "const": 3.14159
+}
+{{</schema>}}
+
+{{<instance-pass `An instance matching the const value is valid`>}}
+3.14159
+{{</instance-pass>}}
+
+{{<instance-fail `An instance not matching the const value is invalid.`>}}
+"pi"
+{{</instance-fail>}}
+
+{{<schema `Schema with a fixed object structure`>}}
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "const": { "name": "John Doe", "age": 30 }
+}
+{{</schema>}}
+
+{{<instance-fail `An empty object is invalid`>}}
+{}
+{{</instance-fail>}}
+
+{{<instance-pass `An instance matching the exact object structure is valid`>}}
+ { "name": "John Doe", "age": 30 }
+{{</instance-pass>}}
+
+{{<instance-fail `An instance not matching the exact object structure is invalid`>}}
+ { "name": "Robert", "age": 30 }
+{{</instance-fail>}}