Auto merge of #80396 - ehuss:fix-missing-deny, r=Mark-Simulacrum
Auto merge of #80396 - ehuss:fix-missing-deny, r=Mark-Simulacrum

Fix missing deny-by-default.md file.

I don't know why, but #80296 deleted this file.  Add it back so that the docs can be viewed directly with mdbook without it auto-generating the file.

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust