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/desserts/pumpkin-pie.md

@@ -0,0 +1,15 @@
+*Source: Abby*
+
+## Ingredients
+- Pie crust
+- Pumpkin puree (fresh or canned)
+- Eggs
+- Heavy cream or evaporated milk
+- Sugar or sweetener
+- Cinnamon
+- Ginger
+- Nutmeg
+- Cloves
+- Salt
+
+**See also:** [Pumpkin Pie from Scratch](https://farmhouseharvest.net/pumpkin-pie-from-scratch/)