docs: simplify branch management to main-only workflow

Author: tw93

CONTRIBUTING.md

@@ -4,18 +4,7 @@
 
 ## Branch Management
 
-```mermaid
-graph LR
-    b_dev(dev) --> b_main(main);
-    contributions([Develop / Pull requests]) -.-> b_dev;
-```
-
-- `dev` branch
-  - `dev` is the developing branch.
-  - It's **RECOMMENDED** to commit feature PR to `dev`.
-- `main` branch
-  - `main` is the release branch, we will make tag and publish version on this branch.
-  - If it is a document modification, it can be submitted to this branch.
+All development happens directly on `main`. Submit pull requests to `main`.
 
 ## Development Setup