Update index.md with table of contents (#20441)

### Description
File previously included TOC placeholder instead of actual TOC



### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

docs/install/index.md

@@ -15,11 +15,10 @@ Details on OS versions, compilers, language versions, dependent libraries, etc c
 under [Compatibility](../reference/compatibility).
 
 ## Contents
-
 {: .no_toc }
 
 * TOC placeholder
-  {:toc}
+{:toc}
 
 ## Requirements