Heroku uses the referrer URL to point at the repo that should be deployed; from this page that includes part of a path that breaks the deployment (specifically /blob/master/docs/Running-Mastodon/Heroku-guide.md).

I've replaced the vanilla address with one that includes a specific reference to the root of the repo

Add separate sections for Heroku and Vagrant deployments. Add guide for instance administration. Move 'Contribution guide' to 'Development guide'.