📝 add more API documentation

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

@@ -0,0 +1,29 @@
+# basic_json::get_binary
+
+```cpp
+binary_t& get_binary();
+
+const binary_t& get_binary() const;
+```
+
+Returns a reference to the stored binary value.
+
+## Return value
+
+Reference to binary value.
+
+## Exception safety
+
+Strong exception safety: if an exception occurs, the original value stays intact.
+
+## Exceptions
+
+Throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if the value is not binary
+
+## Complexity
+
+Constant.
+
+## Version history
+
+- Added in version 3.8.0.