feat(tvix): add mdbook to shell

With this, you can `mdbook build` inside the docs/ directory.

Change-Id: I97df12ed839e3c7a8065563c039436e1a830281e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11243
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/shell.nix

@@ -25,11 +25,14 @@ pkgs.mkShell {
     pkgs.go
     pkgs.grpcurl
     pkgs.hyperfine
+    pkgs.mdbook
+    pkgs.mdbook-plantuml
     pkgs.nix_2_3 # b/313
     pkgs.pkg-config
     pkgs.rust-analyzer
     pkgs.rustc
     pkgs.rustfmt
+    pkgs.plantuml
     pkgs.protobuf
   ] ++ pkgs.lib.optionals pkgs.stdenv.isDarwin [
     # We need these two dependencies in the ambient environment to be able to