[Enhancement] Update Styling

[ddbaptiste]

What does this PR do?

• Adds updated styling via custom.scss and _brand.yml
• Adds an empty references.bib, so that rendering without adding any citations doesn't raise errors.
• Notable visual changes to HTML output:
  • Adds table of contents, by default, to the left side of the document, removing visuals due to "active" (they don't appear to work without some extra JS?)
  • Adds more emphasis to source code
  • Adds a border and heading indicating code output
  • Adds dark mode, toggle
  • Adds more rules to tables, emphasis to table headings, and adds a bit of color

What Wrike task is this associated with?

https://www.wrike.com/open.htm?id=4342774936

Checklist before merging

• If adding a core feature, I've added related tests. (N/A)
• This is part of a product update, and I've added an explanation of what is different to the changelog. (N/A)