Remove @extref from non-Documenter output

Recent versions of Documenter use `@extref` link to documentation of
external packages.

Remove these in the same way and under the same conditions as `@ref` and
`@id` tags.

CHANGELOG.md

@@ -5,6 +5,15 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+ - Literate will now remove Documenter-style `@extref` links under the same
+   circumstances as `@ref` and `@id`.  Recent versions of Documenter use
+   `@extref` to links to the documentation of external packages.  See
+   https://github.com/JuliaDocs/Documenter.jl/issues/2366 for details of
+   the Documenter implementation.
+
 ## [v2.17.0] - 2024-04-14
 ### Added
  - Literate can now output [Quarto](https://quarto.org/) notebooks (markdown documents with

docs/src/documenter.md

@@ -31,8 +31,8 @@ if we set `documenter = true`:
   ````
 
 ### [`Literate.notebook`](@ref):
-- Documenter style `@ref`s and `@id` will be removed. This means that you can use
-  `@ref` and `@id` in the source file without them leaking to the notebook.
+- Documenter style `@ref`s, `@extref`s and `@id` will be removed. This means that you can use
+  `@ref`, `@extref` and `@id` in the source file without them leaking to the notebook.
 - Documenter style markdown math
   ````
   ```math
@@ -57,5 +57,5 @@ if we set `documenter = true`:
   ```
 
 ### [`Literate.script`](@ref):
-- Documenter style `@ref`s and `@id` will be removed. This means that you can use
-  `@ref` and `@id` in the source file without them leaking to the script.
+- Documenter style `@ref`s, `@extref`s and `@id` will be removed. This means that you can use
+  `@ref`, `@extref` and `@id` in the source file without them leaking to the script.

src/Literate.jl

@@ -217,9 +217,11 @@ function replace_default(content, sym;
 
     # Run some Documenter specific things
     if !isdocumenter(config)
-        ## - remove documenter style `@ref`s and `@id`s
+        ## - remove documenter style `@ref`s, `@extref`s and `@id`s
         push!(repls, r"\[([^]]+?)\]\(@ref\)"s => s"\1")     # [foo](@ref) => foo
         push!(repls, r"\[([^]]+?)\]\(@ref .*?\)"s => s"\1") # [foo](@ref bar) => foo
+        push!(repls, r"\[([^]]+?)\]\(@extref\)"s => s"\1")     # [foo](@extref) => foo
+        push!(repls, r"\[([^]]+?)\]\(@extref .*?\)"s => s"\1") # [foo](@extref bar) => foo
         push!(repls, r"\[([^]]+?)\]\(@id .*?\)"s => s"\1")  # [foo](@id bar) => foo
     end