feat: email templates

adds email templates using `react-email` which will be used for invites,
signing and document completion.

authored by @dephraiim

packages/email/tsconfig.json

@@ -0,0 +1,5 @@
+{
+  "extends": "@documenso/tsconfig/react-library.json",
+  "include": ["."],
+  "exclude": ["dist", "build", "node_modules"]
+}