feat!: Split site into public and private sections

Moved the Copper Chronicle and resume into a private git submodule
(src/private), visible only to authenticated users. Added a lock symbol
to the mdbook sidebar for restricted sections. Reorganized nginx/nix
configuration files into a dedicated folder. Removed one-line descriptions
from the menu. Reduced custom CSS to better align the main website's theme
with mdbook's native styling. Personalized the index and blog introduction
pages.

src/recipes/slow-cooker/taco-soup.md

@@ -0,0 +1,12 @@
+*Source: Abby*
+
+## Ingredients
+- Ground beef or turkey
+- Black beans
+- Pinto beans
+- Corn
+- Diced tomatoes
+- Green chilies
+- Taco seasoning
+- Beef broth
+- Toppings: cheese, sour cream, avocado