Improve accessibility by nesting bottom footnotes inside a footer element

[vilhelmgray]

This adds semantic meaning to the footnotes organization and improves accessibility by aiding disabled users who rely on assistive devices such as screen readers which utilize semantic tags such as the footer element in order to navigate properly.

The hr element is semantically defined as representing a paragraph-level thematic break. Now that the footnotes are descendants of the footer element, the hr element originally preceding the footnotes list is no longer necessary (footnotes are no longer paragraph-level content) and thus replaced. However, the footer element is given the class "footnotes" to allow for styling, so the following CSS could be used to provide a stylistically equivalent visible border separating the footnotes from the content if so desired:

    .footnotes {
      border-top: 2px groove gray;
    }

Zola Discourse link: https://zola.discourse.group/t/improve-accessibility-by-nesting-bottom-footnotes-inside-a-footer-element/2356

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Code changes

• Are you doing the PR on the next branch?

• Have you created/updated the relevant documentation page(s)?

This update does not require any interaction from the user to make use of it. However, if would be helpful, I could make a documentation change to mention the visible border CSS suggestion to users.

[Keats]

Thanks!