docs: update project setup commands in READMEs (#3334)

## Summary
- Update init command in `README.md` from `deno run -A -r
https://fresh.deno.dev` to `deno run -Ar jsr:@fresh/init` to match Fresh
2.x
- Fix `www/README.md` to use `deno task dev` (Vite dev server) instead
of `deno task start` (production server) for development workflow

---------

Co-authored-by: Marvin Hagemeister <marvinhagemeister50@gmail.com>
Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>

Author: 3 people

www/README.md

@@ -7,10 +7,10 @@ This is the Fresh website source. The Fresh website contains:
 
 ### Usage
 
-Start the project:
+Start the development server:
 
 ```
-deno task start
+deno task dev
 ```
 
 This will watch the project directory and restart as necessary.