Documentation skeleton (#1617)

* Documentation skeleton

* Added ADR list and more

* rm material theme

* Update mkdocs.yml

* Add docs TOC

* Work for plain MD and mkdocs

* Update mkdocs.yml

* Restructuring

* Added roadmap

* Add reference APIs

* move publishing

docs/overview/architecture-terminology.md

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+# Architecture and Terminology
+
+Backstage is constructed out of three parts. We separate Backstage in this way
+because we see three groups of contributors that work with Backstage in three
+different ways.
+
+- Core - Base functionality built by core devs in the open source project.
+- App - The app is an instance of a Backstage app that is deployed and tweaked.
+  The app ties together core functionality with additional plugins. The app is
+  built and maintained by app developers, usually a productivity team within a
+  company.
+- Plugins - Additional functionality to make your Backstage app useful for your
+  company. Plugins can be specific to a company or open sourced and reusable. At
+  Spotify we have over 100 plugins built by over 50 different teams. It has been
+  very powerful to get contributions from various infrastructure teams added
+  into a single unified developer experience.