doc: collapse installation sections

as suggested in #133

Author: ryantm

README.md

@@ -7,18 +7,10 @@
 * [Problem and solution](#problem-and-solution)
 * [Features](#features)
 * [Installation](#installation)
-  * [niv](#install-via-niv) (Current recommendation)
-    * [module](#install-module-via-niv)
-    * [CLI](#install-cli-via-niv)
+  * [niv](#install-via-niv)
   * [nix-channel](#install-via-nix-channel)
-    * [module](#install-module-via-nix-channel)
-    * [CLI](#install-cli-via-nix-channel)
   * [fetchTarball](#install-via-fetchtarball)
-    * [module](#install-module-via-fetchtarball)
-    * [CLI](#install-cli-via-fetchTarball)
   * [flakes](#install-via-flakes)
-    * [module](#install-module-via-flakes)
-    * [CLI](#install-cli-via-flakes)
 * [Tutorial](#tutorial)
 * [Reference](#reference)
   * [`age` module reference](#age-module-reference)
@@ -56,8 +48,13 @@ Choose one of the following methods:
 * [fetchTarball](#install-via-fetchTarball)
 * [flakes](#install-via-flakes)
 
+<details>
+<summary>
+
 ### Install via [niv](https://github.com/nmattia/niv)
 
+</summary>
+
 First add it to niv:
 
 ```ShellSession
@@ -84,8 +81,15 @@ To install the `agenix` binary:
 }
 ```
 
+</details>
+
+<details>
+<summary>
+
 ### Install via nix-channel
 
+</summary>
+
 As root run:
 
 ```ShellSession
@@ -113,8 +117,15 @@ To install the `agenix` binary:
 }
 ```
 
+</details>
+
+<details>
+<summary>
+
 ### Install via fetchTarball
 
+</summary>
+
 #### Install module via fetchTarball
 
 Add the following to your configuration.nix:
@@ -152,8 +163,15 @@ To install the `agenix` binary:
 }
 ```
 
+</summary>
+
+<details>
+<summary>
+
 ### Install via Flakes
 
+</summary>
+
 #### Install module via Flakes
 
 ```nix
@@ -192,6 +210,8 @@ but, if you want to (change the system based on your system):
 }
 ```
 
+</details>
+
 ## Tutorial
 
 1. The system you want to deploy secrets to should already exist and