Links to project files result in 404

[radeusgd]

General Summary

We sometimes link to source files directly (in contrast to linking to documentation files), for example when mentioning some project configuration files.

These links do not work currently in the documentation (but they work correctly when viewed on GitHub). It would be great if they would also work in the docs, probably linking to the source code in the main branch.

Steps to Reproduce

1. Go to https://dev.enso.org/docs/enso/CONTRIBUTING.html#system-requirements
2. Click on the build.sbt link

Expected Result

I see the file contents of build.sbt file.

Actual Result

Nothing happens at all, if I open it in a new tab I get a 404 error.

[joenash]

Just to note that this is a result of jekyll-relative-links and submodules: jekyll-relative-links converts our relative links to work with the output of static site generation, but we don't actually pull in these project files to the static site, so it's generating links that don't exist.
This PR fixes it by changing those links to absolute ones. But another approach would be to excluse those links from the jekyll-relative-links config, I think, I'm not 100% how it'll work given those files don't exist in the site tree but I'm hoping it excludes just based on the strings.