Update category page UI

[tynandebold]

Following the completion of #120 via #151, let's take another look at the design for the category pages.

Here's what they look like now:

• Post with single categories: https://kedro.org/blog/category/Kedro%20news
• Post with multiple categories: https://kedro.org/blog/category/Databricks

Perhaps we should make the hero section more appealing, or allow for the inclusion of a features post?