feat: add build command

jwebsite/cli.py

@@ -1,6 +1,16 @@
-from click import command
+from pathlib import Path
+
+from click import group
+
+from jwebsite.site import Site
 
 
-@command
-def main() -> None:
-    print("Hello world")
+@group
+def main() -> None: ...
+
+
+@main.command
+def build() -> None:
+    cwd = Path.cwd()
+    with open(cwd / "site.py", encoding="utf-8") as site_config:
+        exec(site_config.read(), {"site": Site(cwd)})