The website (https://monogatari.io/) always has Here you will have the choice between the stable version and the Nightly version. Given that you are on the documentation for version 2.0, nightly is the version you want.
2. Dowloading it from GitHub
3. Dowloading it from a Release Announcement
The GitHub page. https://github.com/Monogatari/Monogatari There you can choose between the different branches. For the newest version of Monogatari, you will want the "Development" branch. This will always be the most up to date version. This version also contains a bunch of source code that won't be necessary to make a game in monogatari, but might be useful to you if you'd like to help contribute to the core engine.
Should you get the version from the main website, or the version linked in news announcements, you simply need to unzip the files and you're ready to go! If you instead opt to get the version on the GitHub page, you'll need to unzip the folder, and then take the contents of the dist folder.