Make site description more descriptive

The site description currently is "GHCup documentation". I believe it would be more helpful to have a description of what GHCup is instead since the description is what shows up in auto-generated embeds on platforms such as Discord and Twitter.

Example:

mkdocs.yml

 site_name: GHCup
 site_url: https://www.haskell.org/ghcup
-site_description: GHCup documentation
+site_description: GHCup is an installer for the general purpose language Haskell.
 site_author: GHCup Team
 site_favicon: haskell_logo.png