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/camping/protein-gunk.md

@@ -0,0 +1,14 @@
+*Source: Abby*
+
+## Description
+High-protein no-bake bars for the trail.
+
+## Ingredients
+- Protein powder
+- Nut butter
+- Honey or dates
+- Oats
+- Nuts and seeds
+- Optional: chocolate chips
+
+See also: [Backpacker's Shepherd's Pie](https://byucampcooking.blogspot.com/2014/03/backpackers-shepherds-pie.html?m=1) (external link)