Migrate four documentation pages to use the new frontend system as the
primary content, moving the old frontend system instructions to --old
files following the established convention:
- configure-app-with-plugins: rewrite for auto-discovered plugins
- customize-theme: rewrite for ThemeBlueprint/extension-based theming
- enable-public-entry: make new frontend system the primary content
- quickstart-app-plugin: remove manual sidebar wiring instructions
Each main file gets an info banner linking to the --old variant, and
each --old file gets a banner linking back to the current guide.
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor
Patrik Oldsberg