Explain the documentation architecture

[roberth]

Observations

Some contributors are skeptical about the nix.dev separation.

• doc: Rename to Nixpkgs reference manual and restate purpose nixpkgs#272183 (comment)

It would be nice for that information to be discoverable, and easy for anyone (but especially us) to refer to.

Problem

We might have to repeat this, and some might not even ask.

Approaches

Willing to help?

As a reviewer.
I did not come up with the policy and do not feel very strongly about it.

Priorities

Add 👍 to issues you find important.