Document deployment slots

[lindydonna]

Move the content from Daria's blog post to a new article. The article can be short and link to App Service, but it should explain the differences in slots for Functions vs. App Service.

[pragnagopa]

@dariagrigoriu - is this addressed? if not can you please open an issue in the doc repo

[ColbyTresness]

@ahmedelnably will write this doc at some point.