Added missing content and improved layout

## What does this MR do?

Restores lost content that was removed from the University landing page.
Adds many videos, presentations and blog articles that were never in University.
Improves the layout of the content.

## Are there points in the code the reviewer needs to double check?

I'd like feedback on the content organisation, specifically if certain sub sections should be moved e.g. from Beginner to Intermediate. Keep in mind how this change will affect the relative difficulty of the other content.

## Why was this MR needed?

The layout looked unprofessional and was missing crucial content.

## What are the relevant issue numbers?

Closes #23051

See merge request !6741