Reorganize directories (#3462)

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

docs/mkdocs/docs/api/basic_json/is_discarded.md

@@ -0,0 +1,72 @@
+# <small>nlohmann::basic_json::</small>is_discarded
+
+```cpp
+constexpr bool is_discarded() const noexcept;
+```
+
+This function returns `#!cpp true` for a JSON value if either:
+
+- the value was discarded during parsing with a callback function (see [`parser_callback_t`](parser_callback_t.md)), or
+- the value is the result of parsing invalid JSON with parameter `allow_exceptions` set to `#!cpp false`; see
+  [`parse`](parse.md) for more information.
+
+## Return value
+
+`#!cpp true` if type is discarded, `#!cpp false` otherwise.
+
+## Exception safety
+
+No-throw guarantee: this member function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+!!! note "Comparisons"
+
+    Discarded values are never compared equal with [`operator==`](operator_eq.md). That is, checking whether a JSON
+    value `j` is discarded will only work via:
+    
+    ```cpp
+    j.is_discarded()
+    ```
+    
+    because
+    
+    ```cpp
+    j == json::value_t::discarded
+    ```
+    
+    will always be `#!cpp false`.
+
+!!! note "Removal during parsing with callback functions"
+
+    When a value is discarded by a callback function (see [`parser_callback_t`](parser_callback_t.md)) during parsing,
+    then it is removed when it is part of a structured value. For instance, if the second value of an array is discarded,
+    instead of `#!json [null, discarded, false]`, the array `#!json [null, false]` is returned. Only if the top-level
+    value is discarded, the return value of the `parse` call is discarded.
+
+This function will always be `#!cpp false` for JSON values after parsing. That is, discarded values can only occur
+during parsing, but will be removed when inside a structured value or replaced by null in other cases.
+
+## Examples
+
+??? example
+
+    The following code exemplifies `is_discarded()` for all JSON types.
+    
+    ```cpp
+    --8<-- "examples/is_discarded.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/is_discarded.output"
+    ```
+
+## Version history
+
+- Added in version 1.0.0.