Rollup merge of #146195 - nixxo:urlencoding-fix, r=ehuss

fix partial urlencoded link support

Hello Rust community.
This is my first contribution, hope is useful.

While translating in Italian the rust book https://github.com/nixxo/rust-lang-book-it I noticed that the linkchecker tool was failing reporting broken links on some pages even if the link worked properly in the browser. Upon inspection I noticed that mdbook basically urlencoded the links, but not urlencoded the heading IDs resulting in a non-identical anchor/IDs pairing that linkchecker reports as non-valid.

looking at the source code for the linkchecker tool I noticed that urlencoding was done by the `small_url_encode` function in a partial way, as the name suggests. Replacing this function with a full urlencoding fixes the issue and the links are properly reported as valid.

- added full urlencoding to properly check urlencoded anchor links against non-urlencoded heading IDs
- added tests

urlecoding provided by https://crates.io/crates/urlencoding

Cargo.lock

@@ -2167,6 +2167,7 @@ version = "0.1.0"
 dependencies = [
  "html5ever",
  "regex",
+ "urlencoding",
 ]
 
 [[package]]
@@ -5824,6 +5825,12 @@ dependencies = [
  "percent-encoding",
 ]
 
+[[package]]
+name = "urlencoding"
+version = "2.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "daf8dba3b7eb870caf1ddeed7bc9d2a049f3cfdfae7cb521b087cc33ae4c49da"
+
 [[package]]
 name = "utf-8"
 version = "0.7.6"