doc: fix build littering doc/

mdbook has the unfortunate habit of creating stub files for chapters it
can't find on disk. turn off this helpful feature as it masks errors in
the summary file, and fix a recently introduced instance of this error.

Change-Id: I10d86aac0489c9c494bd5c8a50047415f4d4b18d

doc/manual/book.toml

@@ -1,6 +1,9 @@
 [book]
 title = "Nix Reference Manual"
 
+[build]
+create-missing = false
+
 [output.html]
 additional-css = ["custom.css"]
 additional-js = ["redirects.js"]

doc/manual/src/SUMMARY.md

@@ -154,7 +154,6 @@
     - [nix run](command-ref/new-cli/nix3-run.md)
     - [nix search](command-ref/new-cli/nix3-search.md)
     - [nix shell](command-ref/new-cli/nix3-shell.md)
-    - [nix show-config](command-ref/new-cli/nix3-show-config.md)
     - [nix store](command-ref/new-cli/nix3-store.md)
     - [nix store add-file](command-ref/new-cli/nix3-store-add-file.md)
     - [nix store add-path](command-ref/new-cli/nix3-store-add-path.md)