REFACTOR: clean the root of the repo (#9231)

# Description
i've almost always wanted to clean up the root of the repo, so here is
my take at it, with some important advice given by @fdncred 😌

- `README.release.txt` is now gone and directly inline in the
`release-pkg` script used in the `release` *workflow*
- `build.rs` has been moved to `scripts/` and its path has been changed
in
[`Cargo.toml`](https://github.com/amtoine/nushell/blob/refactor/clean-root/Cargo.toml#L3)
according to the [*Build Scripts*
section](https://doc.rust-lang.org/cargo/reference/build-scripts.html#build-scripts)
of *The Cargo Book*
- i've merged `images/` into `assets/` and fix the only mention to the
GIF in the README
- i've moved the `docs/README.md` inside the main `README.md` as a new
[*Configuration*
section](https://github.com/amtoine/nushell/tree/refactor/clean-root#configuration)
- the very deprecated `pkg_mgrs/` has been removed
- all the `.nu`, `.sh`, `.ps1` and `.cmd` scripts have been moved to
`scripts/`

### things i've left as-is
- all the other `.md` documents
- the configuration files
- all the Rust and core stuff
- `docker/`
- `toolkit.nu`
- the `wix/` diretory which appears to be important for `winget`

# User-Facing Changes
scripts that used to rely on the paths to some of the scripts should now
call the scripts inside `scripts/` => i think this for the greater good,
it was not pretty nor scalable to have a bunch of scripts in the root of
our main `nushell` 😱

*i even think we might want to move these scripts outside the main
`nushell` repo*
maybe to `nu_scripts` or some other tool 👍 

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- ⚫ `toolkit test`
- ⚫ `toolkit test stdlib`

# After Submitting
```
$nothing
```

scripts/README.md

@@ -0,0 +1,27 @@
+## run the scripts
+
+> **Note**  
+> the following table must be read as follows:
+> - an `x` means *it works*
+> - a `?` means *no data available*
+>
+> `.nu` scripts must be run as `nu .../foo.nu`  
+> `.sh` scripts must be run as `./.../foo.sh`  
+> `.ps1` scripts must be run as `powershell .../foo.ps1`
+>
+> let's say a script is called `foo`
+> - an `x` in the *`./scripts`* column means *`foo` can be run from `./scripts`*
+> - an `x` in the *root* column means *`foo` can be run from the root of `nushell`*
+> - an `x` in the *anywhere* column means *`foo` can be run from anywhere!*
+
+| script                  | `./scripts/` | root | anywhere |
+| ----------------------- | ------------ | ---- | -------- |
+| `build-all-maclin.sh`   | x            | x    | x        |
+| `build-all-windows.cmd` | ?            | x    | ?        |
+| `build-all.nu`          | x            | x    | x        |
+| `coverage-local.nu`     | x            | x    | x        |
+| `coverage-local.sh`     | x            | x    | x        |
+| `install-all.ps1`       | ?            | x    | ?        |
+| `install-all.sh`        | x            | x    | x        |
+| `register-plugins.nu`   | x            | x    | x        |
+| `uninstall-all.sh`      | x            | x    | x        |