[Identity] Document how to use msal-node and msal-browser directly #15573
Description
While we work on improvements for our Identity story, we should document how users could leverage MSAL packages directly to use our SDKs. We’ll need to add information on how to create a TokenCredential that would resolve the getToken method with a token retrieved using @azure/msal-node and @azure/ msal-browser.
We have a sample talking about creating custom TokenCredentials (similar to how .NET also has). Still, we should specifically mention the cases where it will make sense to use MSAL directly with custom TokenCredentials and why.