📝 add more API documentation

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

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+# basic_json::at
+
+```cpp
+// (1)
+reference at(size_type idx);
+const_reference at(size_type idx) const;
+
+// (2)
+reference at(const typename object_t::key_type& key);
+const_reference at(const typename object_t::key_type& key) const;
+
+// (3)
+reference at(const json_pointer& ptr);
+const_reference at(const json_pointer& ptr) const;
+```
+
+1. Returns a reference to the element at specified location `idx`, with bounds checking.
+2. Returns a reference to the element at with specified key `key`, with bounds checking.
+3. Returns a reference to the element at with specified JSON pointer `ptr`, with bounds checking.
+
+## Parameters
+
+`idx` (in)
+:   index of the element to access
+
+`key` (in)
+:   object key of the elements to remove
+    
+`ptr` (in)
+:   JSON pointer to the desired element
+    
+## Return value
+
+1. reference to the element at index `idx`
+2. reference to the element at key `key`
+3. reference to the element pointed to by `ptr`
+
+## Exceptions
+
+1. The function can throw the following exceptions:
+    - Throws [`type_error.304`](../../home/exceptions.md#jsonexceptiontype_error304) if the JSON value is not an array;
+      in this case, calling `at` with an index makes no sense. See example below.
+    - Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if the index `idx` is out of
+      range of the array; that is, `idx >= size()`. See example below.
+2. The function can throw the following exceptions:
+    - Throws [`type_error.304`](../../home/exceptions.md#jsonexceptiontype_error304) if the JSON value is not an object;
+      in this case, calling `at` with a key makes no sense. See example below.
+    - Throws [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403) if the key `key` is is not
+      stored in the object; that is, `find(key) == end()`. See example below.
+3. The function can throw the following exceptions:
+    - Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in the passed
+      JSON pointer `ptr` begins with '0'. See example below.
+    - Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in the passed
+      JSON pointer `ptr` is not a number. See example below.
+    - Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if an array index in the passed
+      JSON pointer `ptr` is out of range. See example below.
+    - Throws [`out_of_range.402`](../../home/exceptions.md#jsonexceptionout_of_range402) if the array index '-' is used
+      in the passed JSON pointer `ptr`. As `at` provides checked access (and no elements are implicitly inserted), the
+      index '-' is always invalid. See example below.
+    - Throws [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403) if the JSON pointer describes a
+      key of an object which cannot be found. See example below.
+    - Throws [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) if the JSON pointer `ptr` can
+      not be resolved. See example below.
+
+## Exception safety
+
+Strong exception safety: if an exception occurs, the original value stays intact.
+
+## Complexity
+
+1. Constant
+2. Logarithmic in the size of the container.
+3. Constant
+
+## Example
+
+??? example
+
+    The example below shows how array elements can be read and written using `at()`. It also demonstrates the different
+    exceptions that can be thrown.
+    
+    ```cpp
+    --8<-- "examples/at__size_type.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/at__size_type.output"
+    ```
+
+??? example
+
+    The example below shows how array elements can be read using `at()`. It also demonstrates the different exceptions
+    that can be thrown.
+        
+    ```cpp
+    --8<-- "examples/at__size_type_const.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/at__size_type_const.output"
+    ```
+
+??? example
+
+    The example below shows how object elements can be read and written using `at()`. It also demonstrates the different
+    exceptions that can be thrown.
+        
+    ```cpp
+    --8<-- "examples/at__object_t_key_type.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/at__object_t_key_type.output"
+    ```
+
+??? example
+
+    The example below shows how object elements can be read using `at()`. It also demonstrates the different exceptions
+    that can be thrown.
+        
+    ```cpp
+    --8<-- "examples/at__object_t_key_type_const.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/at__object_t_key_type_const.output"
+    ```
+
+??? example
+
+    The example below shows how object elements can be read and written using `at()`. It also demonstrates the different
+    exceptions that can be thrown.
+        
+    ```cpp
+    --8<-- "examples/at_json_pointer.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/at_json_pointer.output"
+    ```
+
+??? example
+
+    The example below shows how object elements can be read using `at()`. It also demonstrates the different exceptions
+    that can be thrown.
+        
+    ```cpp
+    --8<-- "examples/at_json_pointer_const.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/at_json_pointer_const.output"
+    ```
+
+## Version history
+
+1. Added in version 1.0.0.
+2. Added in version 1.0.0.
+3. Added in version 2.0.0.