Colocate images and other assets

[senekor]

closes #1574

Rendered

[senekor]

This technically breaks permalinks to the images, but I don't think those are important.

[senekor]

The summary is that this structure:

content/post.md
static/images/post/image.png

moved to this structure:

content/post/index.md
content/post/image.png

And the affected relative image links in the posts are fixed.

There are a small number of non-image assets that were also colocated, some Rust annual survey results posts had a bit of custom JS plus some random HTML file.

I tested pretty much every single asset manually (running zola serve).