Change documentation table footnote style

What does this MR do and why?

This MR originates from this discussion. Our current table footnote style might be confusing because it looks like the start of an ordered list.

To avoid this, we could wrap the footnotes in a <small> tag. Unfortunately, this requires to build the list manually because markdown doesn't render properly within the HTML tag.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Visit http://127.0.0.1:3000/help/development/documentation/styleguide/index.md

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