docs(env): add explanations on env variables

.env.example

@@ -1,7 +1,20 @@
+# Supabase API URL, used to query database on client-side.
 NEXT_PUBLIC_SUPABASE_URL=http://127.0.0.1:54321
+
+# Supabase anon key, used to query database on client-side.
 NEXT_PUBLIC_SUPABASE_ANON_KEY=
-SUPABASE_URL=http://127.0.0.1:54321
+
+# Supabase API URL, used to query database on server-side.
+SUPABASE_URL=$NEXT_PUBLIC_SUPABASE_URL
+
+# Supabase service role key, used to query database on server-side.
 SUPABASE_SERVICE_ROLE_KEY=
+
+# GitHub OAuth app client ID, used to connect to GitHub.
 SUPABASE_AUTH_GITHUB_CLIENT_ID=
+
+# GitHub OAuth app client secret, used to connect to GitHub.
 SUPABASE_AUTH_GITHUB_SECRET=
+
+# Origin of the website URL, used to construct absolute URLs.
 ORIGIN=http://localhost:3000