Fix links in Clippy book

Links to README.md files must actually be links to index.md files,
because of the inner workings of `mdbook`.

Also use the latest mdbook version in CI.

.github/workflows/remark.yml

@@ -29,7 +29,7 @@ jobs:
     - name: Install mdbook
       run: |
         mkdir mdbook
-        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.18/mdbook-v0.4.18-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
+        curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.28/mdbook-v0.4.18-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
         echo `pwd`/mdbook >> $GITHUB_PATH
 
     # Run

book/src/development/basics.md

@@ -4,8 +4,8 @@ This document explains the basics for hacking on Clippy. Besides others, this
 includes how to build and test Clippy. For a more in depth description on the
 codebase take a look at [Adding Lints] or [Common Tools].
 
-[Adding Lints]: https://github.com/rust-lang/rust-clippy/blob/master/book/src/development/adding_lints.md
-[Common Tools]: https://github.com/rust-lang/rust-clippy/blob/master/book/src/development/common_tools_writing_lints.md
+[Adding Lints]: adding_lints.md
+[Common Tools]: common_tools_writing_lints.md
 
 - [Basics for hacking on Clippy](#basics-for-hacking-on-clippy)
   - [Get the Code](#get-the-code)

book/src/usage.md

@@ -148,4 +148,4 @@ clippy-driver --edition 2018 -Cpanic=abort foo.rs
 > that are not optimized as expected, for example.
 
 [Installation]: installation.md
-[CI]: continuous_integration/README.md
+[CI]: continuous_integration/index.md