Document how redirects work in the website

[cnunciato]

We currently support several ways of redirecting:

• Client-side redirects with JavaScript (e.g., to redirect anchored links to full URLs)
• Hugo meta-refreshes that are later converted into S3 301s
• Hugo meta-refreshes that aren't converted into 301s (e.g., temporary redirects)
• Lambda@Edge redirects with regexes for bulk content moves

We should document how each of these works so contributors can choose the right option.