Add footnote support to Markdown

[greatislander]

Is your feature request related to a problem?

The ability to add footnotes to Markdown content should be supported across our sites.

Describe the solution you'd like

The extended syntax for footnotes should be supported via the markdown-it-footnote plugin.

Describe alternative solutions you've considered

Not applicable.

Additional context or notes

• http://dirtystylus.com/2020/06/15/eleventy-markdown-and-footnotes/
• https://github.com/markdown-it/markdown-it-footnote

[greatislander]

Instead, we should recommend the use of https://github.com/KittyGiraudel/eleventy-plugin-footnotes.