Link external functions to external manuals and APIs?

I really don't want to have to put the Drupal Core files into my repo (because then I will have to maintain it) but I would still like the links for Drupal functions to work.
Would it be possible to create a feature similar to how you can link functions to the PHP manual, except so I can link to other manuals/APIs such as api.drupal.org, drupalcontrib.org, freestylesystems.co.uk, etc...