Docs: Rename Posts to Articles in Guides' Getting Started App

This is a continuation of #15215 & #13774.

Recently, the guides have been updated so that all uses of the word, 'Post' (when not referring to an HTTP request) were replaced with 'Article'.

This continues that work by updating the sample application in `code/getting_started` so that it properly aligns with the Getting Started guide and its current usage of the word Article.