emscripten docs: reformat examples to use admonition

* `export EMCC_DEBUG=2` is nice for debugging

* The build artifact cache in `~/.emscripten` sometimes creates issues and needs to be removed from time to time

## Declarative usage {#declarative-usage}

## Examples {#declarative-usage}

Let's see two different examples from `pkgs/top-level/emscripten-packages.nix`:

* The cache at `~/.emscripten` requires to set `HOME=$TMPDIR` in individual phases.

  This makes compilation slower but also more deterministic.

::: {.example #usage-1-pkgs.zlib.override}

See the `zlib` example:

# Using `pkgs.zlib.override {}`

    zlib = (pkgs.zlib.override {

This example uses `zlib` from Nixpkgs, but instead of compiling **C** to **ELF** it compiles **C** to **JavaScript** since we were using `pkgs.zlib.override` and changed `stdenv` to `pkgs.emscriptenStdenv`.

A few adaptions and hacks were put in place to make it work.

One advantage is that when `pkgs.zlib` is updated, it will automatically update this package as well.

```nix

(pkgs.zlib.override {

  stdenv = pkgs.emscriptenStdenv;

}).overrideAttrs

(old: rec {

      --replace 'AR="libtool"' 'AR="ar"' \

      --replace 'ARFLAGS="-o"' 'ARFLAGS="-r"'

  '';

    });

})

```

### Usage 2: pkgs.buildEmscriptenPackage {#usage-2-pkgs.buildemscriptenpackage}

:::{.example #usage-2-pkgs.buildemscriptenpackage}

This `xmlmirror` example features a emscriptenPackage which is defined completely from this context and no `pkgs.zlib.override` is used.

# Using `pkgs.buildEmscriptenPackage {}`

    xmlmirror = pkgs.buildEmscriptenPackage rec {

This `xmlmirror` example features an Emscripten package that is defined completely from this context and no `pkgs.zlib.override` is used.

```nix

pkgs.buildEmscriptenPackage rec {

  name = "xmlmirror";

  buildInputs = [ pkg-config autoconf automake libtool gnumake libxml2 nodejs openjdk json_c ];

  checkPhase = ''

  '';

    };

}

```

:::

### Declarative debugging {#declarative-debugging}

## Debugging {#declarative-debugging}

Use `nix-shell -I nixpkgs=/some/dir/nixpkgs -A emscriptenPackages.libz` and from there you can go trough the individual steps. This makes it easy to build a good `unit test` or list the files of the project.