Update mdbook

This updates mdbook to 0.4.52, which includes a number of fixes and
enhancements since 0.4.48.

Changelog: https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0452

Author: ehuss

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
     if: github.repository == 'rust-lang/rustc-dev-guide'
     runs-on: ubuntu-latest
     env:
-      MDBOOK_VERSION: 0.4.48
+      MDBOOK_VERSION: 0.4.52
       MDBOOK_LINKCHECK2_VERSION: 0.9.1
       MDBOOK_MERMAID_VERSION: 0.12.6
       MDBOOK_OUTPUT__LINKCHECK__FOLLOW_WEB_LINKS: ${{ github.event_name != 'pull_request' }}