docs: Document GitHub App credentials GitHub Enterprise Base URL when creating repository (argoproj#22389)

Add CLI command to Add a private Git repository in GitHub Enterprise and update instruction.

Signed-off-by: Denis Karpelevich <[email protected]>

Author: dkarpele

docs/user-guide/private-repositories.md

@@ -119,13 +119,16 @@ Using the CLI:
 argocd repo add https://github.com/argoproj/argocd-example-apps.git --github-app-id 1 --github-app-installation-id 2 --github-app-private-key-path test.private-key.pem
 ```
 
+!!!note
+    To add a private Git repository on GitHub Enterprise using the CLI add `--github-app-enterprise-base-url https://ghe.example.com/api/v3` flag.
+
 Using the UI:
 
 1. Navigate to `Settings/Repositories`
 
     ![connect repo overview](../assets/repo-add-overview.png)
 
-2. Click `Connect Repo using GitHub App` button, enter the URL, App Id, Installation Id, and the app's private key.
+2. Click `Connect Repo using GitHub App` button, choose type: `GitHub` or `GitHub Enterprise`, enter the URL, App Id, Installation Id, and the app's private key. Enter the GitHub Enterprise Base URL for type `GitHub Enterprise`.
 
     ![connect repo](../assets/repo-add-github-app.png)