Update site description

mkdocs.yml

 site_name: "MARTINS.js: GPU-accelerated Augmented Reality for the web"
 site_url: https://alemart.github.io/martins-js
 site_author: Alexandre Martins
-site_description: Create Augmented Reality experiences that run in web browsers. No need to download apps. WebAR technology.
+site_description: Free and open-source WebAR engine based on Speedy Vision.
 copyright: Copyright © 2022 - present Alexandre Martins
 
 repo_name: alemart/martins-js