Research the ability to add footnote capabilities so we can add footnotes to the guides-source

This issue is a placeholder for some research that is required to look into adding footnotes. This is not purely a technical issue, we need to investigate the effect on writing style if we were to add this functionality. We also need to be mindful of the portability of the Markdown if we're using a custom extension that isn't available in all flavours of markdown.

### Useful links
Showdown extensions currently defined: https://github.com/ember-learn/guidemaker-ember-template/blob/2032bad25c8c4e7429721a1371307ab0896ad55b/app/templates/components/guides-article.hbs#L19
potential footnote extension: https://github.com/Kriegslustig/showdown-footnotes

### Architectural notes
It's probably not sensible that the showdown extensions are actually defined or used in any way from the template 🤔 it is probably worth investigating a way to make this a guidemaker concern before adding to the existing extensions. Some examples might be to have a custom component in guidemaker that deals with markdown rendering (and setups all the correct addons)

Start with app/templates/components/guides-article.hbs and review the Showdown extensions linked in the issue. Investigate the potential footnote extension, Markdown portability, writing-style implications, and how Markdown rendering could become a Guidemaker concern. Done means documenting the trade-offs and a recommended integration approach.

Written by the indexing model from the issue text.