Use better semantic structure for the website

[leouieda]

As @aguspesce rightly pointed out, the current website is mostly organised using <div> which is terrible for accessibility. Instead, we should be using `<section

and

` and be careful about placement of headings. See https://developer.mozilla.org/en-US/docs/Learn/HTML/Introduction_to_HTML/Document_and_website_structure

Any help with this would be more than welcome!