feat: add Copier how-tos

docs/best-practice/webapps.md

docs/deployment/index.md

docs/deployment/secrets.md

docs/deployment/sql-instances.md

docs/deployment/templates.md

docs/deployment/terraform.md

docs/deployment/web-applications.md

docs/explanations/webapp-boilerplate.md

docs/howtos/copier.md

docs/howtos/create-a-new-app.md

docs/reference/cloud-platform/dns.md

mkdocs.yml