Upgrade to Etherpad 3.3, Debian Trixie and Node 24

Summary:
The build instructions have been modified, as Etherpad now uses pnpm to install
dependencies. Instead of build Node, we start with a Node image directly.

The entrypoint is restricted to write settings.json when not yet existing.
Environment checks are skipped when settings.json already exists.

Recent Etherpad versions don't support abiword anymore,
switch to LibreOffice Writer for documents export.

Test Plan: deployed for Naasqueron/Wolfplex instance

Differential Revision: https://devcentral.nasqueron.org/D4229