Add spec for blog format

[johanbove]

Gwil's Garden has a nice "blog" format which deserves more adoption. Therefor we should make it an official format.

Source: https://github.com/sgwilym/gwil-garden/blob/main/src/garden_extensions/blog_posts/extension.tsx

[sgwilym]

Funnily enough I'd like to adopt the log format for my blog once I have it worked out 😁