Move the pinning overview to the Nix manual

[fricklerhandwerk]

Problem

There are currently two pages that should be one.
Since they are reference documentation specific to Nix, they should be in the Nix manual.

Related but different: #907

Approaches
Also add examples.

Willing to help?
As always.

Priorities

Add 👍 to issues you find important.

[olafklingt]

I started related work here #303 i did not get very far but it might provide some input to whoever works on this.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-10-23-documentation-team-meeting-notes-88/34562/1