📝 add more API documentation

doc/mkdocs/docs/api/basic_json/operator""_json.md

@@ -0,0 +1,32 @@
+# basic_json::operator""_json
+
+```cpp
+json operator "" _json(const char* s, std::size_t n)
+```
+
+This operator implements a user-defined string literal for JSON objects. It can be used by adding `#!cpp _json` to a
+string literal and returns a [`json`](../json.md) object if no parse error occurred.
+
+## Parameters
+
+`s` (in)
+:   a string representation of a JSON object
+
+`n` (in)
+:   length of string `s`
+
+## Return value
+
+[`json`](../json.md) value parsed from `s`
+
+## Exceptions
+
+The function can throw anything that [`parse(s, s+n)`](parse.md) would throw.
+
+## Complexity
+
+Linear.
+
+## Version history
+
+- Added in version 1.0.0.