Reorganize directories (#3462)

* 🚚 move files
* 🚚 rename doc folder to docs
* 🚚 rename test folder to tests

docs/mkdocs/docs/features/enum_conversion.md

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+# Specializing enum conversion
+
+By default, enum values are serialized to JSON as integers. In some cases this could result in undesired behavior. If an
+enum is modified or re-ordered after data has been serialized to JSON, the later de-serialized JSON data may be
+undefined or a different enum value than was originally intended.
+
+It is possible to more precisely specify how a given enum is mapped to and from JSON as shown below:
+
+```cpp
+// example enum type declaration
+enum TaskState {
+    TS_STOPPED,
+    TS_RUNNING,
+    TS_COMPLETED,
+    TS_INVALID=-1,
+};
+
+// map TaskState values to JSON as strings
+NLOHMANN_JSON_SERIALIZE_ENUM( TaskState, {
+    {TS_INVALID, nullptr},
+    {TS_STOPPED, "stopped"},
+    {TS_RUNNING, "running"},
+    {TS_COMPLETED, "completed"},
+})
+```
+
+The [`NLOHMANN_JSON_SERIALIZE_ENUM()` macro](../api/macros/nlohmann_json_serialize_enum.md) declares a set of
+`to_json()` / `from_json()` functions for type `TaskState` while avoiding repetition and boilerplate serialization code.
+
+## Usage
+
+```cpp
+// enum to JSON as string
+json j = TS_STOPPED;
+assert(j == "stopped");
+
+// json string to enum
+json j3 = "running";
+assert(j3.get<TaskState>() == TS_RUNNING);
+
+// undefined json value to enum (where the first map entry above is the default)
+json jPi = 3.14;
+assert(jPi.get<TaskState>() == TS_INVALID );
+```
+
+## Notes
+
+Just as in [Arbitrary Type Conversions](arbitrary_types.md) above,
+
+- [`NLOHMANN_JSON_SERIALIZE_ENUM()`](../api/macros/nlohmann_json_serialize_enum.md) MUST be declared in your enum type's
+  namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to
+  integer serialization.
+- It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.
+
+Other Important points:
+
+- When using `get<ENUM_TYPE>()`, undefined JSON values will default to the first pair specified in your map. Select this
+  default pair carefully.
+- If an enum or JSON value is specified more than once in your map, the first matching occurrence from the top of the
+  map will be returned when converting to or from JSON.