doc/beam: update docs for mixRelease frontend deps

##### mixRelease - Frontend dependencies (javascript) {#mix-release-javascript-deps}

For phoenix projects, inside of Nixpkgs you can either use yarn2nix (mkYarnModule) or node2nix. An example with yarn2nix can be found [here](https://github.com/NixOS/nixpkgs/blob/master/pkgs/servers/web-apps/plausible/default.nix#L39). An example with node2nix will follow. To package something outside of nixpkgs, you have alternatives like [npmlock2nix](https://github.com/nix-community/npmlock2nix) or [nix-npm-buildpackage](https://github.com/serokell/nix-npm-buildpackage)

For phoenix projects, inside of Nixpkgs you can either use `fetchYarnDeps` or `buildNpmPackage`. An example with `fetchYarnDeps` can be found [here](https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/pl/plausible/package.nix). An example with `fetchYarnDeps` will follow. To package something outside of nixpkgs, you have alternatives like [npmlock2nix](https://github.com/nix-community/npmlock2nix) or [nix-npm-buildpackage](https://github.com/serokell/nix-npm-buildpackage)

##### mixRelease - backend dependencies (mix) {#mix-release-mix-deps}