docs: fix internal links and add user-facing SQL templating guide

- Fix broken links in quickstart.mdx pointing to old /docs/installation
  and /docs/configuration paths, now redirecting to /admin-docs/
- Fix broken links in faq.mdx for installation, configuration, and
  contributing paths
- Fix link in generate-database-docs.mjs for auto-generated database
  index page
- Add new user-facing SQL templating guide in using-superset/ that
  covers practical usage of Jinja templates in SQL Lab
- Add cross-link tip in admin SQL templating config guide pointing
  users to the new user guide

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/scripts/generate-database-docs.mjs

@@ -548,7 +548,7 @@ Superset to a database is to **install the proper database driver(s)** in your e
 You'll need to install the required packages for the database you want to use as your metadata database
 as well as the packages needed to connect to the databases you want to access through Superset.
 For information about setting up Superset's metadata database, please refer to
-installation documentations ([Docker Compose](/docs/installation/docker-compose), [Kubernetes](/docs/installation/kubernetes))
+installation documentations ([Docker Compose](/admin-docs/installation/docker-compose), [Kubernetes](/admin-docs/installation/kubernetes))
 :::
 
 ## Supported Databases