PubGrub guide

Source for the PubGrub guide. The published version is available at https://pubgrub-rs-guide.pages.dev. This guide is made with mdBook. To compile it locally, install mdBook and run

mdbook serve

We use prettier for consistent formatting. The easiest way to run it is through npx:

npx prettier --write --print-width 80 --prose-wrap always "**/*.md"
npx prettier --write "**/*.yml"