Improve hover UX for internal site links

[okjuan]

• [x] make the hover-link pseudo-element bigger
• [x] don't open hovercard when cursor is not near (or very close to) hyperlink
• [x] prevent hovercards from appearing on mobile
• [x] position hovercard above link if link is near bottom of viewport
• [x] similar to the above, position hovercard to the left if link is near right of viewport