Update mdbook to 0.4.48

This brings in several updates. Two significant ones are to halve the
search index size, and the other introduces major changes to footnote
rendering.

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

Author: ehuss

src/doc/rustc/src/SUMMARY.md

@@ -102,7 +102,7 @@
     - [s390x-unknown-linux-gnu](platform-support/s390x-unknown-linux-gnu.md)
     - [s390x-unknown-linux-musl](platform-support/s390x-unknown-linux-musl.md)
     - [sparc-unknown-none-elf](./platform-support/sparc-unknown-none-elf.md)
-    - [sparcv9-sun-solaris](platform-support/solaris.md)
+    - [solaris](platform-support/solaris.md)
     - [\*-pc-windows-gnullvm](platform-support/pc-windows-gnullvm.md)
     - [\*-nto-qnx-\*](platform-support/nto-qnx.md)
     - [\*-unikraft-linux-musl](platform-support/unikraft-linux-musl.md)
@@ -126,7 +126,6 @@
     - [\*-win7-windows-msvc](platform-support/win7-windows-msvc.md)
     - [x86_64-fortanix-unknown-sgx](platform-support/x86_64-fortanix-unknown-sgx.md)
     - [x86_64-pc-cygwin](platform-support/x86_64-pc-cygwin.md)
-    - [x86_64-pc-solaris](platform-support/solaris.md)
     - [x86_64-unknown-linux-none.md](platform-support/x86_64-unknown-linux-none.md)
     - [x86_64-unknown-none](platform-support/x86_64-unknown-none.md)
     - [xtensa-\*-none-elf](platform-support/xtensa.md)

src/tools/rustbook/Cargo.lock

@@ -880,9 +880,9 @@ dependencies = [
 
 [[package]]
 name = "mdbook"
-version = "0.4.47"
+version = "0.4.48"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7e1a8fe3a4a01f28dab245c474cb7b95ccb4d3d2f17a5419a3d949f474c45e84"
+checksum = "8b6fbb4ac2d9fd7aa987c3510309ea3c80004a968d063c42f0d34fea070817c1"
 dependencies = [
  "ammonia",
  "anyhow",

src/tools/rustbook/Cargo.toml

@@ -15,6 +15,6 @@ mdbook-i18n-helpers = "0.3.3"
 mdbook-spec = { path = "../../doc/reference/mdbook-spec" }
 
 [dependencies.mdbook]
-version = "0.4.47"
+version = "0.4.48"
 default-features = false
 features = ["search"]