doc: improve backend intro doc

src/backend/README.md

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 # Puter Backend
 
-- [Documentation for Contributors](./doc/contributors/index.md)
+_Part of a High-Level Distributed Operating System_
+
+Whether or not you call Puter an operating system
+(we call it a "high-level distributed operating system"),
+**operating systems for devices** 
+are a useful reference point to describe the architecture of Puter.
+If Puter's "hardware" is services, and Puter's "userspace" is the
+client side of the API, then Puter's "kernel" is the backend.
+
+Puter's backend is composed of:
+- The **Kernel** class, which is responsible for initialization
+- A number of **Modules** which are registered in **Kernel** for a customized
+  Puter instance.
+- Many **Services** which are contained inside modules.
+
+## Documentation
+
+- [Backend File Structure](./doc/contributors/structure.md)
+- [Boot Sequence](./doc/contributors/boot-sequence.md)
+- [Kernel](./doc/Kernel.md)
+- [Modules](./doc/contributors/modules.md)
+
+## Can I use Puter's Backend Alone?
+
+Puter's backend is not dependent on Puter's frontned. In fact, you could
+prevent Puter's GUI from ever showing up by disabling PuterHomepageModule.
+Similarly, you can run Puter's backend with no modules loaded for a completely
+blank slate, or only include CoreModule and WebModule to quickly build your
+own backend that's compatible with any of Puter's services.
+
+## What can it do?
+
+Puter's Kernel only initializes modules, nothing more. The modules bring a lot
+of capabilities to the table, however. Within this directory you'll find modules that:
+- coerce all the well-known AI services to a common interface
+- manage authentication with Wisp servers (this brings TCP to the browser!)
+- manage apps on Puter
+- allow a user to host websites from Puter
+- provide persistent key-value storage to Puter's desktop and apps
+- provide a fast filesystem implementation
+- communicate with other instances of Puter's backend,
+  secured with elliptic curve cryptography
+- provide more services like converting files and compiling low-level code.
+
+![diagram of Puter backend connections](./doc/assets/puter-backend-map.drawio.png)

src/backend/doc/README.md

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+## Backend - Contributor Documentation
+
+### Where to Start
+
+Start with [Backend File Structure](./contributors/structure.md).
+
+### Index
+
+- [Backend File Structure](./contributors/structure.md)
+- [Boot Sequence](./contributors/boot-sequence.md)
+- [Kernel](./Kernel.md)
+- [Modules](./contributors/modules.md)