docs: explain self-hosting, content import and safe upgrades

.env.example

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+# Exact public origin; use HTTPS behind your reverse proxy in production.
+SITE_URL=http://localhost:3000
+# Native development/maintenance database and uploads directory.
+DATA_DIR=./data
+# Optional. If absent, initialization generates a secret in DATA_DIR/auth-secret.
+# BETTER_AUTH_SECRET=replace-with-at-least-32-random-characters
+# For compose.selfhost.yaml, directory mounted at /data:
+PORTFOLIO_DATA_DIR=./data