Add links to docs for every feature in the /features/ page

Closes https://gitlab.com/gitlab-com/www-gitlab-com/issues/1306 - for https://gitlab.com/gitlab-com/www-gitlab-com/issues/1302

It took longer than expected bc I had to add styles for the links.

Review apps link: https://add-links-to-features-page.about.gitlab.com/features/