docs: Introduce an user guide (#1121)

docs/user-guide/install.md

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+# Installation
+
+You must use a `nixpkgs` version compatible with the nixvim version you choose.
+
+The `master` branch requires to use a _very recent_ version of nixpkgs unstable.
+In order to guarantee the compatibility between nixvim & nixpkgs it is recommended to always update both at the same time.
+
+When using a `stable` version you must use the corresponding nixvim branch, for example `nixos-23.11` when using NixOS 23.11.
+
+Failure to use the correct branch, or an old revision of nixpkgs will likely result in errors of the form `vimPlugins.<name> attribute not found`.
+
+NixVim can be used in four ways:
+- As a NixOS module
+- As a Home-Manager module
+- As a nix-darwin module
+- As a standalone derivation
+
+NixVim is also available for nix flakes, or directly through an import.
+
+## Accessing nixvim
+
+For a direct import you can add nixvim to your configuration as follows:
+```nix
+let 
+    nixvim = import (builtins.fetchGit {
+        url = "https://github.com/nix-community/nixvim";
+        # When using a different channel you can use `ref = "nixos-<version>"` to set it here
+    });
+in
+```
+
+When using flakes you can simply add `nixvim` to the inputs:
+```nix
+{
+    inputs.nixvim = {
+        url = "github:nix-community/nixvim";
+        # If using a stable channel you can use `url = "github:nix-community/nixvim/nixos-<version>"`
+        inputs.nixpkgs.follows = "nixpkgs";
+    };
+
+    # outputs...
+}
+```
+
+## Usage as a module (NixOS, Home-Manager, nix-darwin)
+
+When using NixVim as a module you must import the NixVim module into your module system.
+The three imports are:
+- `<nixvim>.homeManagerModules.nixvim`
+- `<nixvim>.nixosModules.nixvim`
+- `<nixvim>.nixDarwinModules.nixvim`
+
+`<nixvim>` refers to the way to access nixvim, depending on how you fetched nixvim as described in the previous section.
+
+The imports can be added as a `imports = [ <nixvim_import> ]` in a configuration file.
+
+You will then be able to enable nixvim through `programs.nixvim.enable = true`, and configure the
+options as `programs.nixvim.<path>.<to>.<option> = <value>`.
+
+## Standalone usage
+
+When using nixvim as a standalone derivation you can use the following functions, located in `<nixvim>.legacyPackages.${system}`:
+- `makeNixvim`: This function takes an attribute set of options values as arguments
+- `makeNixvimWithModule`: This function takes an attribute set of the form: `{pkgs, extraSpecialArgs, module}`.
+  The only required argument is `module`, being a NixOS module. This gives access to the `imports`, `options`, `config` variables, and using functions like `{config, ...}: { ... }`.
+
+There are also some helper functions in `<nixvim>.lib.${system}` like:
+- `check.mkTestDerivationFromNixvimModule`, taking the same arguments as `makeNixvimWithModule` and generates a check derivation.
+- `check.mkTestDerivationFromNvim`, taking an attribute set of the form `{name = "<derivation name>"; nvim = <nvim derivation>}`. The `nvim` is the standalone derivation provided by NixVim.
+
+The nixvim derivation can then be used like any other package!
+
+For more information you can look at the [nixvim standalone flake template](https://github.com/nix-community/nixvim/blob/main/templates/simple/flake.nix).