Refactoring of auth content (#106)

There used to be just one authentication.md file under concepts, now there's an authentication root at authentication/authentication.md.

Will leave this branch to be merged with master too. There will be a small merge conflict but since this is a non-destructive change (if the redirect doesn't work the 404 error still occurs) it's easy to fix.

msteams-platform/redirects/authentication.md

@@ -0,0 +1,3 @@
+---
+redirect_url: /microsoftteams/platform/concepts/authentication/authentication
+---