Add the composer command for installing WxT 6.3.0 in the installation guide

Problem/Motivation

I notice that the Installation guide is missing the composer command to install WxT 6.3.0:

composer create-project drupalwxt/site-wxt:11.3.x-dev <site-name> --no-interaction

Steps to reproduce

N/A

Proposed resolution

Update the Installation guide to add the installation command, and update the Drupal 11 - alpha release label to Drupal 11 release since the WxT 6.1.0 and WxT 6.3.0 are no longer alpha releases (I believe)

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A