📝 overworked documentation

Replacing references to std exceptions with user-defined exceptions.
Also changed some examples to the new exceptions.

doc/examples/json_pointer.output

@@ -1,3 +1,3 @@
-domain_error: JSON pointer must be empty or begin with '/'
-domain_error: escape error: '~' must be followed with '0' or '1'
-domain_error: escape error: '~' must be followed with '0' or '1'
+[json.exception.parse_error.107] parse error at 1: JSON pointer must be empty or begin with '/' - was: 'foo'
+[json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'
+[json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'