Redirect old urls to new

With the new IA some old urls need to be redirected to new ones.

Octicons

• https://primer.style/octicons/ → https://primer.style/foundations/icons/
• https://primer.style/octicons/guidelines/design → https://primer.style/foundations/icons/design-guidelines
• https://primer.style/octicons/:icon-name → https://primer.style/foundations/icons/:icon-name