Update commands for building docs in CONTRIBUTING.md (#14945)

- `$DOCS_FORMAT` is no longer supported in Makefile
- By default HTML and EPUB doc formats are created

Author: eksperimental

CONTRIBUTING.md

@@ -184,19 +184,20 @@ 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 run:
+# Now we will go back to Elixir's root directory,
+cd "${elixir_dir}"
 
-```sh
-make docs                  # to generate HTML pages
-make docs DOCS_FORMAT=epub # to generate EPUB documents
+# and generate HTML and EPUB documents:
+make docs
 ```
 
 This will produce documentation sets for `elixir`, `eex`, `ex_unit`, `iex`, `logger`,