Docs: documentation on supported authentication is not current

[dtivel]

Documentation says:

However, vsce now supports other authentication strategies like managed identities.

[lszomoru]

@dtivel, we can certainly go ahead and update the vsce documentation to mention the --azure-credential flag, but we would also need to describe the steps on how to grant permissions to a managed identity to the Marketplace publisher. Does the Marketplace team have any documentation for that? Are there any 1st party docs that we can publish?