Step-by-Step Evaluation of IFD Expression with Sequence Diagram

3rd chunk of `doc/manual/source/language/import-from-derivation.md`

239c9b7361e4abe3473d3043e2ec9ff324b6f857b293361b0000000100000cde

|          |           |             |                        |
|          V           |             |                        |
|    .------------.    |             |                        |
|    |   value    |    |             |                        |
|    '------------'    |             |                        |
+----------------------+             +------------------------+
```

In more detail, the following sequence diagram shows how the expression is evaluated step by step, and where evaluation is blocked to wait for the build output to appear.

```
.-------.     .-------------.                        .---------.
|Nix CLI|     |Nix evaluator|                        |Nix store|
'-------'     '-------------'                        '---------'
    |                |                                    |
    |evaluate IFD.nix|                                    |
    |--------------->|                                    |
    |                |                                    |
    |  evaluate `"${readFile drv} world"`                 |
    |                |                                    |
    |    evaluate `readFile drv`                          |
    |                |                                    |
    |   evaluate `drv` as string                          |
    |                |                                    |
    |                |instantiate /nix/store/...-hello.drv|
    |                |----------------------------------->|
    |                :                                    |
    |                :  realise /nix/store/...-hello.drv  |
    |                :----------------------------------->|
    |                :                                    |
    |                                                     |--------.
    |                :                                    |        |
    |      (evaluation blocked)                           |  echo hello > $out
    |                :                                    |        |
    |                                                     |<-------'
    |                :        /nix/store/...-hello        |
    |                |<-----------------------------------|
    |                |                                    |
    |  resume `readFile /nix/store/...-hello`             |
    |                |                                    |
    |                |   readFile /nix/store/...-hello    |
    |                |----------------------------------->|
    |                |                                    |
    |                |               hello                |
    |                |<-----------------------------------|
    |                |                                    |
    |      resume `"${"hello"} world"`                    |
    |                |                                    |
    |        resume `"hello world"`                       |
    |                |                                    |
    | "hello world"  |                                    |
    |<---------------|                                    |
.-------.     .-------------.                        .---------.
|Nix CLI|     |Nix evaluator|                        |Nix store|
'-------'     '-------------'                        '---------'
```

Title: Step-by-Step Evaluation of IFD Expression with Sequence Diagram

Summary

This section details the step-by-step evaluation of an IFD expression using a sequence diagram involving the Nix CLI, Nix evaluator, and Nix store. The diagram illustrates the process of evaluating the expression, including instantiating a derivation, realizing it in the store, and reading the resulting file content. It also highlights the point at which the evaluation is blocked while waiting for the store object to be built, and shows the data flow between the components as the evaluation resumes and completes.