Add shortcode/component for code links

Documentation of a software should be able to link (at least roughly) to where a given feature is implemented in the software source repository, both as a means to see more details for those who want it, and as a celebration of open source / encouragement to contribute.

The existing `service` shortcode is a specialisation of that, but it could be nice to support a more general `source` shortcode that defaults to using a base of the software source repo, as a variable specified in the global config file, together with a path to the component in question. That might include a repo icon (e.g. Octocat for GitHub), and possibly some descriptive text.

Taking inspiration from popular software library docs like numpy, a code-styled link (e.g. [`[source]`]()) may be a reasonable approach :-)

Start by locating the existing `service` shortcode and the global config file, then examine how shortcode links are rendered in the theme. Define the source shortcode's repository base, component path, and optional presentation, and verify that generated documentation links to the intended source locations.

Written by the indexing model from the issue text.