Rollup merge of rust-lang#95954 - AnthonyMikh:fix-broken-coverage-docs-screenshot-link, r=Dylan-DPC

Fix broken link in coverage tools docs

During stabilization the link to example screenshot wad not updated, making rendered docs somewhat less useful. Move that screenshot from unstable book into rustc docs and make documentation point to that new place. Also remove `/img` in unstable book since there are no more any files there.

Author: Dylan-DPC

src/doc/unstable-book/src/compiler-flags/img/llvm-cov-show-01.png renamed to src/doc/rustc/src/images/llvm-cov-show-01.png

@@ -145,7 +145,7 @@ $ llvm-cov show -Xdemangler=rustfilt target/debug/examples/formatjson5 \
     -name=add_quoted_string
 ```
 
-<img alt="Screenshot of sample `llvm-cov show` result, for function add_quoted_string" src="img/llvm-cov-show-01.png" class="center"/>
+<img alt="Screenshot of sample `llvm-cov show` result, for function add_quoted_string" src="images/llvm-cov-show-01.png" class="center"/>
 <br/>
 <br/>