Fix instructions for building docs from release sources

The following instructions do not work because the source code version
of the release create a folder with the version in it.

```console
❯ unzip -l v1.19.3.zip
Archive:  v1.19.3.zip
427d21e
  Length      Date    Time    Name
---------  ---------- -----   ----
        0  11-13-2025 11:33   elixir-1.19.3/
      502  11-13-2025 11:33   elixir-1.19.3/.formatter.exs
...
```

So the instructions do not work because it expects a folder named `elixir/`.

Instructions have been updated to work from the Git repo as well as from the
downloaded release source files.

Author: eksperimental

CONTRIBUTING.md

@@ -177,17 +177,19 @@ affecting **all external contributors**:
 ## Building documentation
 
 Building the documentation requires that [ExDoc](https://github.com/elixir-lang/ex_doc)
-is installed and built alongside Elixir:
+is installed and built alongside Elixir.
+
+After cloning and compiling Elixir, run:
 
 ```sh
-# After cloning and compiling Elixir, in its parent directory:
-git clone https://github.com/elixir-lang/ex_doc.git
-cd ex_doc && ../elixir/bin/elixir ../elixir/bin/mix do deps.get + compile
-```
+elixir_dir=$(pwd)
+cd .. && git clone https://github.com/elixir-lang/ex_doc.git
+cd ex_doc && "${elixir_dir}/bin/elixir" "${elixir_dir}/bin/mix" do deps.get + compile
 
-Now go back to Elixir's root directory and generate HTML and EPUB documents:
+# Now we will go back to Elixir's root directory,
+cd "${elixir_dir}"
 
-```sh
+# and generate HTML and EPUB documents:
 make docs
 ```