docs: build with eleventy instead of jekyll

[alxndrsn]

Benefits:

• drop ruby dependency

Eleventy is mostly compatible with Jekyll, so there are minimal changes to the site.

The main changes are:

• collections need to be defined explicitly, and (may) need to change dir
• collections are referenced via collections.x instead of site.x
• collection props are referenced via item.data.x instead of item.x
• page props are renamed from e.g. page.title to global title
• include vs page usage of the posts collection means that post now becomes post.data vs global, depending on how the template is used

TODO

• syntax highlighting