Fix #6742 - use flexbox for front page layout

[havarddj]

This PR changes the structure of the boxes on the home page to use css flexbox instead of styling html tables. This makes it much easier to add content without messing up the layout, or even adding more boxes down the line. It also improves the user experience on small screens.

Before:

After:

(Also fixes some unclosed <a>-tags in the yaml)

[rvisser7]

This looks great, thanks! If I may add a small comment: I'd probably have a slight preference for the flexboxes to be top-aligned. I think it feels a bit easier to visually scan the page when the “A database” and “Announcements” headers line up horizontally (but this is just my personal preference 🙂).

[havarddj]

Good call, I've fixed that and made a couple more tweaks. @assaferan, did you want to review it? Let me know if you have any suggestions! That said, I feel like I've personally reached the point of diminishing returns (partly mostly a skill issue)