Created nlohmann::basicjson::operator[] (markdown)

nlohmann::basicjson::operator[].md

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+```cpp
+reference operator[](size_type idx);                              // 1
+const_reference operator[](size_type idx) const;                  // 2
+```
+```cpp
+reference operator[](const object_t::key_type& key)               // 3
+const_reference operator[](const object_t::key_type& key) const   // 4
+```
+
+## Description
+
+1. Returns a reference to the element at specified index `idx`. No bounds checking is performed. In case `idx > size()`, null values are inserted between `size()` and `idx - 1` (only works for arrays).
+2. Returns a reference to the element at specified index `idx`. No bounds checking is performed and no values will be inserted (only works for arrays).
+3. Returns a reference to the value that is mapped to a key equivalent to `key`, performing an insertion if such key does not already exist. If an insertion is performed, the mapped value is value-initialized (default-constructed for class types, zero-initialized otherwise) and a reference to it is returned. No bounds checking is performed (only works for objects).
+4. Returns a reference to the mapped value of the element with key equivalent to `key`. No bounds checking is performed and no values will be inserted (only works for objects).
+
+## Parameters
+
+- `idx`: index of the element to return
+- `key`: key of the element to return
+
+## Return value
+
+Reference to the specified value.
+
+## Complexity
+
+1. Constant.
+2. Logarithmic in the size of the container.
+
+## Exceptions
+
+1. `std::runtime_error` when called on a JSON container with other type than `array`.
+2. `std::runtime_error` when called on a JSON container with other type than `object`.