Update README.md to enhance Vue 2.x and Vue 3.x examples and clarify …

…usage instructions

- Added missing Vue 2.x example code for ActionCableVue setup.
- Improved clarity on connectionUrl behavior in Vue 3.x.
- Updated channel subscription instructions for Vue 3 setup script.
- Enhanced documentation for subscribing and unsubscribing from channels.
- Added Nuxt.js setup instructions for both Nuxt 2 and Nuxt 3.
- Removed outdated sections and improved overall structure for better readability.