docs: introduction chapter

Adds an introduction chapter to the manual.

docs/manual/introduction.md

@@ -0,0 +1,32 @@
+# Introduction to Home Manager {#ch-introduction}
+
+Home Manager is a [Nix](https://nix.dev/)-powered tool for reproducible management of the contents of users' home directories.
+This includes programs, configuration files, environment variables and, well… arbitrary files.
+The following example snippet of Nix code:
+
+```nix
+programs.git = {
+  enable = true;
+  userEmail = "[email protected]";
+  userName = "joe";
+};
+```
+
+would make available to a user the `git` executable and man pages and a configuration file `~/.config/git/config`:
+
+```ini
+[user]
+	email = "[email protected]"
+	name = "joe"
+```
+
+Since Home Manager is implemented in Nix, it provides several benefits:
+
+- Contents are reproducible — a home will be the exact same every time it is built, unless of course, an intentional change is made.
+  This also means you can have the exact same home on different hosts.
+- Significantly faster and more powerful than various backup strategies.
+- Unlike "dotfiles" repositories, Home Manager supports specifying programs, as well as their configurations.
+- Supported by <http://cache.nixos.org/>, so that you don't have to build from source.
+- If you do want to build some programs from source, there is hardly a tool more useful than Nix for that, and the build instructions can be neatly integrated in your Home Manager usage.
+- Infinitely composable, so that values in different configuration files and build instructions can share a source of truth.
+- Connects you with the [most extensive](https://repology.org/repositories/statistics/total) and [most up-to-date](https://repology.org/repositories/statistics/newest) software package repository on earth, [Nixpkgs](https://github.com/NixOS/nixpkgs).

docs/manual/manual.md

@@ -8,6 +8,7 @@ preface.md
 ```
 
 ```{=include=} parts
+introduction.md
 installation.md
 usage.md
 nix-flakes.md