Add external links to docs

Created by: github-learning-lab[bot]

With the addition of links to our code base, we should add a unit test as one of our Travis CI jobs that run whenever it builds. Let's add a test using HTMLProofer to test for any broken links, images, or HTML.

We'll now add the HTMLProofer test, but we'll also refactor some of the commands away from the config file into standalone test scripts.

Step 8: Apply batch suggested fixes to refactor config

Let's make sure all links are valid by adding a link checker unit test, and packaging into test scripts.

⌨ Activity: Check for broken links using Travis CI

1. Go to the Files changed tab.
2. Next to each suggested change, click Add suggestion to batch.
3. At the top of the page, click Commit suggestions and enter a commit message.
4. Click Commit changes.

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