docs(installation): add advanced installation (#1643)

Adds an "Advanced Installation" page to the documentation website.

docs/installing/README.md

@@ -0,0 +1,52 @@
+# 🚀 Advanced Installation
+
+To install starship, you need to do two things:
+
+1. Get the **starship** binary onto your computer
+1. Tell your shell to use the starship binary as its prompt by modifying its init scripts
+
+For most users, the instructions on [the main page](/guide/#🚀-installation) will work great. However,
+for some more specialized platforms, different instructions are needed.
+
+There are so many platforms out there that they didn't fit into the main
+README.md file, so here are some installation instructions for other platforms
+from the community. Is yours not here? Please do add it here if you figure it
+out!
+
+## [Nix](https://nixos.wiki/wiki/Nix)
+
+### Getting the Binary
+
+#### Imperatively
+
+```sh
+nix-env -iA nixos.starship
+```
+
+#### Declarative, single user, via [home-manager](home-manager)
+
+Add `pkgs.starship` to your `home.packages` in your `home.nix` file, then run
+
+```sh
+home-manager switch
+```
+
+#### Declarative, system-wide, with NixOS
+
+Add `pkgs.starship` to `environment.packages` in your `configuration.nix`,
+then run
+
+```sh
+sudo nixos-rebuild switch
+```
+
+### Modifying Init Scripts
+
+#### With Nix and home-manager, using zsh:
+
+Add the following to `programs.zsh.initExtra` in your `home.nix` file, then
+run
+
+```sh
+home-manager switch
+```