Fix some spelling errors - mostly in comments & documentation.

I did not touch the Changelog file and any third party stuff.

additonal -> additional (1)
apppend -> append (2)
constuctor -> constructor (2)
contect -> context (2)
dobulequote -> doublequote (1)
elemnts -> elements (1)
exakt -> exact (2)
exluded -> exclude (1)
explicitely -> explicitly (2)
narcissic -> narcissistic (1)
ocurred -> occurred (1)
occuring -> occurring (2)
preceeds -> preceded (1)
ot -> to (2)
wehther -> whether (2)

doc/examples/json_pointer__operator_add.cpp

@@ -9,7 +9,7 @@ int main()
     json::json_pointer ptr("/foo");
     std::cout << ptr << '\n';
 
-    // apppend a JSON Pointer
+    // append a JSON Pointer
     ptr /= json::json_pointer("/bar/baz");
     std::cout << ptr << '\n';
 

doc/examples/json_pointer__operator_add_binary.cpp

@@ -8,7 +8,7 @@ int main()
     // create a JSON pointer
     json::json_pointer ptr("/foo");
 
-    // apppend a JSON Pointer
+    // append a JSON Pointer
     std::cout << ptr / json::json_pointer("/bar/baz") << '\n';
 
     // append a string