Migrate docs to developers.home-assistant.io (#4215)

* Migrate docs to developers.home-assistant.io

Move all documentation (except the kernel.md, for which it makes sense to be
kept here) to developers.home-assistant.io.

Just bluetooth.md was intentionally not preserved, as the information value was
low and it was out of date anyway.

See home-assistant/developers.home-assistant#2748

* Fix reference links

Author: sairon

Documentation/README.md

@@ -1,9 +1,9 @@
 # Documentation
 
-## Contents
+Documentation for the Home Assistant Operating System is available at [developers.home-assistant.io/docs/operating-system/][docs].
+If you want to contribute to this documentation, please refer to the [home-assistant/developers.home-assistant][docs-repo] repository.
 
-- [Configuration](./configuration.md) - how users can configure HAOS
-- [Network](./network.md) - approach to networking
-- [Bluetooth](./bluetooth.md) - approach to bluetooth
-- [Kernel](./kernel.md) - kernel versions
-- [Boards](./boards/README.md) - board specific documentation
+For the list of Linux kernel versions used currently in this branch of Home Assistant Operating System, see [kernel.md](./kernel.md).
+
+[docs]: https://developers.home-assistant.io/docs/operating-system/
+[docs-repo]: https://github.com/home-assistant/developers.home-assistant/