Getting started docs

I always forget how to get going with dream2nix, and while this
page doesn't contain much more than you'd already find in the
examples, I do think it's a good starting point for allowing
people to start more confidently.

modules/flake-parts/site/src/SUMMARY.md

@@ -1,5 +1,7 @@
 
 - [Introduction](./intro.md)
+- [Getting started](./gettingstarted.md)
+- [Contact](./contact.md)
 
 # Examples
 - [dream2nix examples](./examples.md)

modules/flake-parts/site/src/contact.md

@@ -0,0 +1,2 @@
+If you run into any problems or would like to discuss the project,
+check out the [#dream2nix:nixos.org](https://matrix.to/#/#dream2nix:nixos.org) Matrix channel.

modules/flake-parts/site/src/gettingstarted.md

@@ -0,0 +1,79 @@
+## Getting started
+
+To package a piece of software with dream2nix, you'd typically start with
+a bare-bones dream2nix flake like this:
+
+```nix
+{
+  description = "My flake a dream2nix package";
+
+  inputs = {
+    dream2nix.url = "github:nix-community/dream2nix";
+    nixpkgs.follows = "dream2nix/nixpkgs";
+  };
+
+  outputs = inputs @ {
+    self,
+    dream2nix,
+    nixpkgs,
+    ...
+  }: let
+    system = "x86_64-linux";
+  in {
+    packages.${system}.default = dream2nix.lib.evalModules {
+      packageSets.nixpkgs = inputs.dream2nix.inputs.nixpkgs.legacyPackages.${system};
+      modules = [
+        ./default.nix
+        {
+          paths.projectRoot = ./.;
+          paths.projectRootFile = "flake.nix";
+          paths.package = ./.;
+        }
+      ];
+    };
+  };
+}
+```
+
+And a `default.nix` that looks like this:
+
+```nix
+{
+  config,
+  lib,
+  dream2nix,
+  ...
+}: {
+  imports = [
+    # dream2nix modules go here
+  ];
+
+  deps = {nixpkgs, ...}: {
+    # dependencies go here
+  };
+
+  name = "my-package-name";
+  version = "2.7.1";
+
+  # Ecosystem-dependent package definition goes here
+}
+```
+
+To find out which dream2nix modules to import, browse through the modules
+on the left and the [examples](./examples.md). When getting started, the
+'single-language' collection will be most helpful. Once you're comfortable
+packaging a single single-language project, you could look into packaging
+a repository containing multiple packages, or a multi-language package.
+
+Once you have imported a module, this module will make ecosystem-dependent
+functions available to create your package definition, such as `mkDerivation`
+or `buildPythonPackage`.
+
+### Building
+
+Now, `git add` your `flake.nix` and `default.nix`. Depending on the module
+you selected, you will generate and commit a `lock.json` describing the
+'pinned' dependencies of your project. The error message produced by
+`nix build .` should tell you how to generate this lock file, likely
+something like `nix run .#default.lock`.
+