Nano's source code is clonable from https://git.savannah.gnu.org/git/nano.git.
It can be browsed on the web at https://git.savannah.gnu.org/cgit/nano.git/log/.
To build and run the bleeding-edge version of GNU nano, read the instructions located in the git repository itself.
The source code of this website is available from https://git.wh0rd.org/nano-editor.git.
See the current versions in some distros.