Minimal, reproducible code sample, a copy-pastable example if possible

~$ cd src/numcodecs/docs

~/src/numcodecs/docs$ make
Makefile:12: *** recipe commences before first target.  Stop.

~/src/numcodecs/docs$ make html
Makefile:12: *** recipe commences before first target.  Stop.

Problem description

Developer without sphinx installed in the environment cannot build the documentation.

Version and installation information

Please provide the following:

• main branch from git
• no sphinx installed