In Doxygen you use reference links: define them separately and then refer to them from within the text.
* This is a documentation. Here I link [std::string] to an external web page.
* The next line is the link definition:
* [std::string]: http://en.cppreference.com/w/cpp/string/basic_string "std::string documentation"
In order to achieve your aim I think your best bet is to make use of the ALIAS facility.
ALIASES += std_string="<a href=\"http://en.cppreference.com/w/cpp/string/basic_string\" title=\"std::string documentation\" target=\"_blank\">std::string</a> "
ALIASES += std_vector="<a href=\"http://en.cppreference.com/w/cpp/container/vector\" title=\"std::vector documentation\" target=\"_blank\">std::vector</a> "
std::string -> http://en.cppreference.com/w/cpp/string/basic_string
std::vector -> http://en.cppreference.com/w/cpp/container/vector
@std_ref std::string string/basic_string
@std_ref std::vector container/vector
It's worth noting that what you are currently using is the notation that comes only with Doxygen's support for Markdown - it's not the doxygen method for external links. The original Doxygen method is to insert an HTML link inline...
<a href="linkURL">link text</a>
... but that makes no difference to your original problem.
In order to achieve your aim I think your best bet is to make use of the ALIAS facility. The relevant manual page is here. Using them, you should be able to define an alias like
std-string and have it insert the HTML link everywhere you use the alias.
ALIASES are set up in the doxyfile config file (in this section of the manual)