📝 add more API documentation

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

@@ -0,0 +1,29 @@
+# basic_json::parse_event_t
+
+```cpp
+enum class parse_event_t : std::uint8_t {
+    object_start,
+    object_end,
+    array_start,
+    array_end,
+    key,
+    value
+};
+```
+
+The parser callback distinguishes the following events:
+
+- `object_start`: the parser read `{` and started to process a JSON object
+- `key`: the parser read a key of a value in an object
+- `object_end`: the parser read `}` and finished processing a JSON object
+- `array_start`: the parser read `[` and started to process a JSON array
+- `array_end`: the parser read `]` and finished processing a JSON array
+- `value`: the parser finished reading a JSON value
+
+## Example
+
+![Example when certain parse events are triggered](../../images/callback_events.png)
+
+## Version history
+
+- Added in version 1.0.0.