fix(ci): update site_url in mkdocs.yml (#467)

Configure MkDocs to use the custom domain: `docs.tagstud.io`

Author: CarterPillow

mkdocs.yml

@@ -11,7 +11,7 @@
 
 site_name: TagStudio
 site_description: "A User-Focused Photo & File Management System"
-site_url: https://tagstudiodev.github.io/tagstudio
+site_url: https://docs.tagstud.io/
 repo_url: https://github.com/TagStudioDev/TagStudio
 edit_uri: blob/main/docs/
 repo_name: TagStudioDev/TagStudio