Fix accessibility issues

[brylie]

There are several accessibility issues with the current site design, including:

• small fonts (10pt font difficult to read on desktop #17, Make text easier to read on desktop #18)
• low contrast (More contrast please #19)
• using tables for layout
• lack of heading structure

A full analysis can be found here:
https://wave.webaim.org/report#/https://news.python.sc/

Let's improve these issues, so the Pythonic News is comfortable and accessible for people to read.