Create footnotes to external references

[lwasser]

In our guidebook review @Midnighter noted that we have a lot of external links to content that will take people away from our guidebook. in many cases it is external information. the comment discussion is below:

#144 (comment)

i think this is a really good point. we might want to use footnotes which are a part of the theme we are using instead.
i'm creating this issue so we can move forward with merging the existing pr while thinking about overall style!

[lwasser]

looking at this in the text, i think footnotes are a great way to keep user focus on the lesson content while also providing additional resources. We will want to go back through the guide and be consistent with using them. But for now we can use them on new lesson pages!

[willingc]

Next action for contributors:

• Go through existing guide content and add footnotes