Development setup : Vagrant is promoted despite not being preferred #1949

As seen in mastodon/mastodon#38019 (comment), the Vagrant setup is broken and is not used by the majority of the team. However the instructions in the documentation present it as the easy "quick start" option. Also, the repository contains a DEVELOPMENT.md document that seems to be more accurate compared to the documentation site.

I therefore suggest updating the recommended development environment in the docs or replacing that with/linking to the DEVELOPMENT document.