Get (links to) "next" and "previous" page

[jplatte]

Just realized that I use this for my blog and it doesn't really fit into the design I've been going with, where pages in a directory are fully parsed and rendered in parallel.

However, since frontmatter reading and content rendering are independent steps, I think it should be possible to adjust the file processing to submit the content rendering work as a background task and return right afterwards, which would allow a function for getting metadata of the next / previous page to block until all metadata for the directory is available. I am not certain how summaries, which are based on the content, fit into this though. I'll create a separate issue for that.

[jplatte]

Done in 903b594 🎉