doc: collapse installation sections

as suggested in #133

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)
@@ -49,15 +41,13 @@ All files in the Nix store are readable by any system user, so it is not a suita
 
 ## Installation
 
-Choose one of the following methods:
-
-* [niv](#install-via-niv)
-* [nix-channel](#install-via-nix-channel)
-* [fetchTarball](#install-via-fetchTarball)
-* [flakes](#install-via-flakes)
+<details>
+<summary>
 
 ### Install via [niv](https:/nmattia/niv)
 
+</summary>
+
 First add it to niv:
 
 ```ShellSession
@@ -84,8 +74,15 @@ To install the `agenix` binary:
 }
 ```
 
+</details>
+
+<details>
+<summary>
+
 ### Install via nix-channel
 
+</summary>
+
 As root run:
 
 ```ShellSession
@@ -113,8 +110,15 @@ 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 +156,15 @@ To install the `agenix` binary:
 }
 ```
 
+</details>
+
+<details>
+<summary>
+
 ### Install via Flakes
 
+</summary>
+
 #### Install module via Flakes
 
 ```nix
@@ -192,6 +203,8 @@ 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