Integrate mdbook-spec for the reference.

This updates the reference which is now using a new mdbook plugin. This
requires a little extra work than a normal book because the plugin uses
`rustdoc` to generate links to the standard library. It also ensures
that the submodule is available for *any* command that uses rustbook,
since it is now part of the rustbook workspace.

src/tools/rustbook/Cargo.lock

@@ -661,6 +661,20 @@ dependencies = [
  "textwrap",
 ]
 
+[[package]]
+name = "mdbook-spec"
+version = "0.1.2"
+dependencies = [
+ "anyhow",
+ "mdbook",
+ "once_cell",
+ "pathdiff",
+ "regex",
+ "semver",
+ "serde_json",
+ "tempfile",
+]
+
 [[package]]
 name = "mdbook-trpl-listing"
 version = "0.1.0"
@@ -794,6 +808,12 @@ dependencies = [
  "windows-targets 0.52.6",
 ]
 
+[[package]]
+name = "pathdiff"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8835116a5c179084a830efb3adc117ab007512b535bc1a21c991d3b32a6b44dd"
+
 [[package]]
 name = "percent-encoding"
 version = "2.3.1"
@@ -1079,6 +1099,7 @@ dependencies = [
  "env_logger",
  "mdbook",
  "mdbook-i18n-helpers",
+ "mdbook-spec",
  "mdbook-trpl-listing",
  "mdbook-trpl-note",
 ]