Rename doc/manual{src -> source}

This is needed to avoid this
https://github.com/mesonbuild/meson/issues/13774 when we go back to
making our subproject directory `src`.

doc/manual/source/quick-start.md

@@ -0,0 +1,43 @@
+# Quick Start
+
+This chapter is for impatient people who don't like reading documentation.
+For more in-depth information you are kindly referred to subsequent chapters.
+
+1. Install Nix:
+
+   ```console
+   $ curl -L https://nixos.org/nix/install | sh
+   ```
+
+   The install script will use `sudo`, so make sure you have sufficient rights.
+
+   For other installation methods, see the detailed [installation instructions](installation/index.md).
+
+1. Run software without installing it permanently:
+
+   ```console
+   $ nix-shell --packages cowsay lolcat
+   ```
+
+   This downloads the specified packages with all their dependencies, and drops you into a Bash shell where the commands provided by those packages are present.
+   This will not affect your normal environment:
+
+   ```console
+   [nix-shell:~]$ cowsay Hello, Nix! | lolcat
+   ```
+
+   Exiting the shell will make the programs disappear again:
+
+   ```console
+   [nix-shell:~]$ exit
+   $ lolcat
+   lolcat: command not found
+   ```
+
+1. Search for more packages on [search.nixos.org](https://search.nixos.org/) to try them out.
+
+1. Free up storage space:
+
+   ```console
+   $ nix-collect-garbage
+   ```