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.

.gitignore

@@ -1,4 +1,4 @@
-blog/
-src
-content/*
-example/*
+# the build directory of mdbook
+book/
+*.jpg
+*.png