docs: add metadata block in @mtkmodel

Author: DhairyaLGandhi

docs/src/basics/MTKLanguage.md

@@ -31,6 +31,7 @@ set to `false`.
   - `@equations`: for the list of equations
   - `@extend`: for extending a base system and unpacking its unknowns
   - `@icon` : for embedding the model icon
+  - `@metadata`: for assigning key-value pairs as model level metadata
   - `@parameters`: for specifying the symbolic parameters
   - `@structural_parameters`: for specifying non-symbolic parameters
   - `@variables`: for specifying the unknowns
@@ -94,6 +95,16 @@ end
         v_for_defaults => 2.0
     end
 end
+
+struct Author end
+struct ModelVersion end
+@mtkmodel ModelD begin
+    @description "A component with some metadata."
+    @metadata begin
+        Author = "Test Author"
+        ModelVersion = "1.0.0"
+    end
+end
 ```
 
 #### `@description`
@@ -257,6 +268,30 @@ using ModelingToolkit
 end
 ```
 
+#### `@metadata` begin block
+
+  - Assign key-value pairs as model level metadata.
+  - The keys must be `DataType` to avoid any key collisions.
+  - Assignments can be made using either `=` or `=>`.
+  - Metadata can be retrieved using [`getmetadata`](@ref).
+  - Metadata can be set using [`setmetadata`](@ref).
+
+```@example mtkmodel-example
+using ModelingToolkit
+
+struct Author end
+struct ModelVersion end
+
+@mtkmodel MetadataModel begin
+    @metadata begin
+        Author = "Test Author"
+        ModelVersion => "1.0.0"
+    end
+end
+@named model = MetadataModel()
+getmetadata(model, Author, nothing) == "Test Author"
+```
+
 #### A begin block
 
   - Any other Julia operations can be included with dedicated begin blocks.