Move setup documentation out of generated package

[egpbos]

The generated package contains documentation for (further) setting up the generated package itself. I think this should be part of the documentation of the template. Specifically:

• next_steps.md should be a section of the template's docs. The message in post_gen_package.py should then give a URL linking to this Next Steps section.
• Same goes for project_setup.md, except that it doesn't need to be referenced at all during generation.
• The Documentation section in README.md should be commented out by default using <!-- blabla --> HTML comments. Then in the readthedocs "next steps" issue, an instruction to uncomment this section and add the link could be be added.

I'm writing this issue as a todo list for myself, but anyone else that wants to pick it up: feel free :)