Add conceptual doc for when to use ChainedTokenCredential with usage guidelines for DefaultAzureCredential

[ahsonkhan]

Create content to be published on Learn, for C++.
For now, we'd host it on our GitHub SDK repo, until there's a new home and point the aka.ms link to it.

We can use the other language docs as templates to start with and modify with C++ specific content.

.NET: https://aka.ms/azsdk/net/identity/credential-chains
Go: https://aka.ms/azsdk/go/identity/credential-chains
Java: https://aka.ms/azsdk/java/identity/credential-chains
JS: https://aka.ms/azsdk/js/identity/credential-chains
Python: https://aka.ms/azsdk/python/identity/credential-chains

cc @scottaddie, @ronniegeraghty

[scottaddie]

Addressed in the linked PR ☝

This doc will publish with the initial C++ SDK content push to Learn.