I'd like to install Magit. According to the Installation chapter of the Magit user manual
Magit is available from Melpa and Melpa-Stable. If you haven’t used Emacs’ package manager before, then it is high time you familiarize yourself with it by reading the documentation in the Emacs manual, see (emacs)Packages.
By default, packages are downloaded from a single package archive maintained by the Emacs developers. This is controlled by the variable package-archives [...] You can alter this list if you wish to use third party package archives—but do so at your own risk, and use only third parties that you think you can trust!
The maintainers of package archives can increase the trust that you can have in their packages by signing them. [...] Package archives should provide instructions on how you can obtain their public key.
Where can I find the MELPA archives' instructions on how I can obtain their public key?