Ensure that the book is built after the doc/ directory.

Without this, rustbook was failing because it was expecting the
directory to exist. (Previously, rustbook was just silently failing to
install the CSS files due to this.)

mk/docs.mk

@@ -283,6 +283,6 @@ compiler-docs: $(COMPILER_DOC_TARGETS)
 
 trpl: doc/book/index.html
 
-doc/book/index.html: $(RUSTBOOK_EXE) $(wildcard $(S)/src/doc/trpl/*.md)
+doc/book/index.html: $(RUSTBOOK_EXE) $(wildcard $(S)/src/doc/trpl/*.md) | doc/
 	$(Q)rm -rf doc/book
 	$(Q)$(RUSTBOOK) build $(S)src/doc/trpl doc/book