Remove absolute links from homepage

Use relative links instead.
Improve HTML on about page

- A document can only have one <main> element
- Fix unclosed <header> tag
- Paragraph tags cannot go inside lists
Tweak words on about page
Remove role="breadcrumbs" from breadcrumbs

I don't think role="breadcrumbs" has ever been a thing.
Remove second main element from homepage

A document can only contain one <main> element.

Re-work the styles so that the specificity still works when
we don't have another main element on the page.
Merge pull request #573 from alphagov/copyedit-homepage-about-page

Improve information pages