[FEATURE REQUEST] Add link to seqsender repo to docs site

[leebrian]

The seqsender doc site is full of useful information. I'm not sure how to get back to the repo from the documentation pages site. Obviously, I can use the back button, but if I get the link through other means it's hard to understand where it's hosted.

Would you please add a link to the main tab or the about tab that goes back to the main repo (ie, https://github.com/cdcgov/seqsender)