Update installation section in README

All dependencies are actually available from apt sources, so it's possible
to skip both pip and gem.  (For some reason this does however not work in
GitHub actions.)

Supplement instructions in README and mention shortcut on Debian/Ubuntu.

Signed-off-by: Joachim Wiberg <[email protected]>

Author: troglobit

README.md

@@ -27,8 +27,13 @@
 2. Install dependencies:
    ```bash
    pip install pyyaml
+   gem install --user-install asciidoctor-pdf rouge
    ```
 
+> [!NOTE]
+> On Debian/Ubuntu systems you can use standard packages for the requirements:
+> `sudo apt install python3-yaml ruby-asciidoctor-pdf ruby-rouge`
+
 ---
 
 ## Usage