Update mdbook to 0.4.48

This updates to the latest version of mdbook which has had a variety
of fixes of new features since the last update.

Changelog: https://github.com/rust-lang/mdBook/blob/master/CHANGELOG.md#mdbook-0448

Author: ehuss

src/doc/rustc-dev-guide/.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
     if: github.repository == 'rust-lang/rustc-dev-guide'
     runs-on: ubuntu-latest
     env:
-      MDBOOK_VERSION: 0.4.21
+      MDBOOK_VERSION: 0.4.48
       MDBOOK_LINKCHECK2_VERSION: 0.9.1
       MDBOOK_MERMAID_VERSION: 0.12.6
       MDBOOK_TOC_VERSION: 0.11.2