📝 add more API documentation

doc/mkdocs/docs/api/basic_json/size.md

@@ -0,0 +1,60 @@
+# basic_json::size
+
+```cpp
+size_type size() const noexcept;
+```
+
+Returns the number of elements in a JSON value.
+    
+## Return value
+
+The return value depends on the different types and is
+defined as follows:
+
+Value type  | return value
+----------- | -------------
+null        | `0`
+boolean     | `1`
+string      | `1`
+number      | `1`
+binary      | `1`
+object      | result of function object_t::size()
+array       | result of function array_t::size()
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant, as long as [`array_t`](array_t.md) and [`object_t`](object_t.md) satisfy
+the Container concept; that is, their `size()` functions have constant
+complexity.
+
+## Notes
+
+This function does not return the length of a string stored as JSON
+value - it returns the number of elements in the JSON value which is `1` in
+the case of a string.
+
+## Example
+
+??? example
+
+    The following code calls `size()` on the different value
+    types.
+    
+    ```cpp
+    --8<-- "examples/size.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/size.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.
+- Extended to return `1` for binary types in version 3.8.0.