ASTRO: Blog styling

[jamesmisson]

• Add styling to the blog.
• BLOG: Per-page config #1682

[LlGC-jop]

I've moved #1657 into in progress and assigned myself as there seems to be some overlap with this one.

[LlGC-jop]

see #1657 for latest on structure

[LlGC-szw]

Tweak the header image and post styling ready for testing.