add docs for quarto inspect extension information (quarto-cli/pulls/12735)

docs/advanced/inspect/index.qmd

@@ -61,6 +61,8 @@ If an additional parameter is given, the output is written to the specified path
     - `source`: the content of the code cell
     - `language`: the language of the code cell
     - `metadata`: the cell-level metadata for the code cell
+- `extensions`: an array of objects containing information about extensions installed in the project.
+  The precise description of the information provided in each object is given by the [TypeScript interface `Extension` in Quarto's source code](https://github.com/quarto-dev/quarto-cli/blob/main/src/extension/types.ts#L29-L44) (We do not expect this interface to change, but we reserve the right to do so if needed).
 
 ## JSON Schemas
 

docs/advanced/inspect/quarto-inspect-project-json-schema.json

@@ -83,7 +83,7 @@
                     }
                 }
             }
-        }
-
+        },
+        "extensions": { "type": "array", "items": { "type": "object" } }        
     }
 }