This documentation uses a format called Org. It’s native to Emacs but the subset of it that is used here is small enough and simple enoug that it can be written in any text editor. To familiarize yourself with Org, take a look at this Org Mode Syntax Cheat Sheet.
Org is native to Emacs, so if you have Emacs, you already have Org setup and ready.
Vim/NVim doesn’t have full support for all of Orgs features, but the subset used here is supported. You can get support by installing org.vim.
VSCode/VSCodium doesn’t have full support for all of Orgs features, but the subset used here is supported. You can get support by installing vscode-org-mode.org-mode.