content: add "updated" field to frontmatter of recently edited pages #2547

What page(s) need to be updated?

• https://web.dev/serve-images-webp/
• https://web.dev/use-imagemin-to-compress-images/

Why is this update needed?

This field was omitted in #2524 when addressing the changes proposed in #2523. Adding it per the template post example in order to keep the revision history accurate.