docs: add link to Travis CI headless page (#38990)

docs(headless): link to Travis CI docs

- Travis has a specific page in its docs all about headless, with several different methods of using `xvfb`
  - c.f. https://docs.travis-ci.com/user/gui-and-headless-browsers/

docs/tutorial/testing-on-headless-ci.md

@@ -32,18 +32,7 @@ xvfb-maybe electron-mocha ./test/*.js
 
 ### Travis CI
 
-On Travis, your `.travis.yml` should look roughly like this:
-
-```yml
-addons:
-  apt:
-    packages:
-      - xvfb
-
-install:
-  - export DISPLAY=':99.0'
-  - Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
-```
+For Travis, see its [docs on using Xvfb](https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-xvfb-to-run-tests-that-require-a-gui).
 
 ### Jenkins