Contiribute documentation referes to makefile targets that don't exist

[nemith]

I was reading through the contibution documenation here https://nornir.readthedocs.io/en/latest/contributing/index.html#starting-development-environment and it mentions a developer environment as well as a docker-compose dependency handling.

It states:

Some tests requires additional services to be running which are managed by docker-compose. You can start these services with:
make start_dev_env

Seems like these were removed in #562 but the documentation wasn't reflected.

[dbarrosop]

Yes, that needs updating. Since we separated core from plugins into different repositories there is no need for docker containers to test the core of nornir