Preprocessing of posts via Jekyll plugin

[joshbeckman]

Could the building of the site be faster if more was done as preprocessing in Jekyll plugins?

• backlinks
• network data
• tags mapping to slugs (https://github.com/joshbeckman/notes/blob/master/_includes/Content.html#L71)
• book/source linking (https://github.com/joshbeckman/notes/blob/master/_includes/Content.html#L84)
• extra links (https://github.com/joshbeckman/notes/blob/master/_includes/Content.html#L2)

[joshbeckman]

Example of setting site metadata: https://github.com/jekyll/github-metadata/blob/main/lib/jekyll-github-metadata/site_github_munger.rb#L94

Also useful for #122