Add URL Checking to Markdown + Doc Files

[mrmundt]

Fixes NA

Summary/Motivation:

Because I like to make our pre-test linting suite as complicated as possible, this adds yet another element: URL checking! The action in use here checks all Markdown and RST files and makes sure that the URLs work. It ignores pyomo-jenkins, however, because that's behind a firewall.

Changes proposed in this PR:

• Check URLs to make sure that they work (in Markdown and RST files)
• Fix broken URLs
• Remove the section on "Including External Packages" because we don't actually support that anymore and the URL for the one example was broken

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[michaelbynum]

I love it! I hate broken links!