docs: encapsulate default config in <details> tags

Author: AntoineGS

README.md

@@ -101,7 +101,8 @@ use {
 }
 ```
 
-The following is the default configuration:
+<details>
+<summary>Default configuration</summary>
 
 ```lua
 require('copilot').setup({
@@ -182,6 +183,8 @@ require('copilot').setup({
 })
 ```
 
+</details>
+
 ### panel
 
 Panel can be used to preview suggestions in a split window. You can run the