Relative links from external markdown files included in the docs are broken

[toolness]

While working on #321 I found that we have about 6 broken links in our docs that are due to including markdown files from the web-design-standards repo.

The original links are relative links to files in the repository which work fine when viewed on GitHub. For example, the page at https://standards.usa.gov/getting-started/code-guidelines/ is just the file https://github.com/18F/web-design-standards/blob/develop/CONTRIBUTING.md fetched and injected into our docs site. It includes a relative link to RELEASE.md, which works fine when viewed on github.com. However, the relative link breaks on standards.usa.gov because its relative URL turns into https://standards.usa.gov/getting-started/code-guidelines/RELEASE.md, which 404s.

I can think of two different solutions here:

1. Ensure that all markdown files included from the other repository only have absolute links in them.
2. Post-process the markdown files to automatically convert their relative URLs into absolute URLs based at github.com, so that they effectively point to the same place they do when viewed on github.

The first option doesn't require writing any code, but it's hard to ensure quality, unless we introduce a test case in the other repo that ensures no relative links exist in the markdown files.

The second option requires writing some ruby code.

Any preference @donjo?

[donjo]

I don't really have a preference though the second option seems a bit more future proof as teams turn over and new people have to take this on.