Markdown footnote improvements

What does this MR do and why?

1. Prevent footnote OL from being reorderable
   The ordered list in markdown footnotes is not reorderable, but we present affordances for it in the UI.
2. Improve styling of footnotes
   Add separator style, adjust font size and margins of list items and paragraphs

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

Hovering:

Before	After

How to set up and validate locally

1. Check out this branch
2. Create an issue in the GDK with footnotes
3. The footnotes will be in a section at the bottom of the description with smaller font, with a small divider line above the list of footnotes
4. Hover over the footnotes. There will not be a grab handle rendered at the left like normal ordered lists in gitlab markdown.

Example markdown content to create footnotes:

I'm creating a footnote[^1] here as an example of how they are currently[^2] styled.

[^1]: This is my example footnote
[^2]: This is another footnote

Related to #26118 (closed), #482771 (closed), #469616 (closed)