Document the derivation build process and sandbox

[roberth]

Problem

Proposal

A new page or reorganized sections on the derivation page that describe

• what building a derivation entails, outline (require inputs' outputs, run builder, check outputs, probably sandbox)
• what is the process in more detail?
  • it requires that dependencies' outputs are available (valid)
  • how is the environment set up
  • run the builder
  • what is the builder expected to do?
  • final checks (refer to the advanced derivation attrs)
  • what effect does recursive nix have? dynamic derivations?
  • anything different for ca? fod?
• what the sandbox is like on each platform
  • which files are available
  • which permissions are available
  • quirks

Not in scope for this issue, but perhaps worth linking:

• Scheduling constraints
• Stdenv concepts
  • the build is virtually always specified by Nixpkgs stdenv, and almost never by you

Place relevant links in the docs touched by (or grep for the number of this issue, 8888)

• reword description of how realisation works #7592

Checklist

• checked latest Nix manual (source)
• checked open documentation issues and pull requests for possible duplicates

Priorities

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