CONTRIBUTING.rst mentions outdated "nox -s docs" command

[salty-horse]

CONTRIBUTING.rst instructs contributors to build the docs with nox -s docs, but this session was removed in #1271.

It is unclear if and how the docs should be regenerated.

[mljoshi]

Is this referring to the API documentation currently found in docs/epy?

[salty-horse]

I don't know what this command is supposed to build, since it was removed.