doc: improve docs organization

- Move extension docs back into this repo
- Add index README.md under `doc/`
- Improve documentation of project structure

doc/README.md

@@ -0,0 +1,24 @@
+## User Documentation
+
+- [Hosting Instructions](./self-hosters/instructions.md)
+- [Domain Setup](./self-hosters/domains.md)
+- [Support Levels](./self-hosters/support.md)
+
+## Contributor Documentation
+
+### Where to Start
+
+Start with [Repo Structure and Tooling](./contributors/structure.md).
+
+### Index
+
+- **Conventions**
+  - [Repo Structure and Tooling](./contributors/structure.md)
+    - How directories and files are organized in our GitHub repo
+    - What tools are used to build parts of Puter
+  - [Comment Prefixes](./contributors/comment_prefixes.md)
+    - A convention we use for line comments in code
+
+- [Frontend Documentation](/src/gui/doc)
+- [Backend Documentation](/src/backend/doc)
+- [Extensions](./contributors/extensions/)