docs(phoenix): add example for elixir phoenix

examples/phoenix/.test.sh

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+#!/bin/sh
+set -ex
+mix local.hex --force
+mix local.rebar --force
+echo Y | mix archive.install hex phx_new
+echo Y | mix phx.new hello
+sed -i.bak -e "s/username: \"postgres\",/socket_dir: System.get_env(\"PGDATA\"),/" \
+    ./hello/config/dev.exs && rm ./hello/config/dev.exs.bak
+devenv up&
+timeout 20 bash -c 'until echo > /dev/tcp/localhost/4000; do sleep 0.5; done'
+curl -s http://localhost:4000/ | grep "Phoenix Framework"

examples/phoenix/README.md

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+Based on [the official tutorial](https://hexdocs.pm/phoenix/installation.html).
+
+```shell-session
+$ devenv shell
+$ mix local.hex --force
+$ mix local.rebar --force
+$ mix archive.install hex phx_new
+$ mix phx.new hello
+$ sed -i.bak -e "s/username: \"postgres\",/socket_dir: System.get_env(\"PGDATA\"),/" \
+    ./hello/config/dev.exs && rm ./hello/config/dev.exs.bak
+$ devenv up
+```

examples/phoenix/devenv.nix

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+{ pkgs, lib, ... }:
+
+{
+  packages = lib.optionals pkgs.stdenv.isLinux [ pkgs.inotify-tools ];
+
+  languages.elixir.enable = true;
+
+  services.postgres.enable = true;
+
+  processes.phoenix.exec = "cd hello && mix ecto.create && mix phx.server";
+}

examples/phoenix/devenv.yaml

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+inputs:
+  nixpkgs:
+    url: github:NixOS/nixpkgs/nixpkgs-unstable