How can I customize my robot framework log.html and output so that I can add some external links to my output files like log.html and output.xml file.
You can use metadata in your test suites. Those metadata can contains external links. This is explained in the "Free test suite metadata" section of the User Guide.
Here is how it looks if you use TXT format for your tests:
*** Settings ***
Metadata Here is a link http://www.external.com/pages.html
And that will be included in your output.xml and report.html