docs: add manual Linux .deb packaging guide

[Supp140106]

What does this PR add?

Adds a step-by-step guide for packaging a Neutralino app as a .deb
file for Linux, intended for users who want to do it manually without
an automated script.

Why is this needed?

Currently, the distribution docs don't cover Linux .deb packaging.
Many Linux users distribute apps via .deb and there's no official
guide on how to structure the package for a Neutralino app.

What does the guide cover?

• Building the app with neu build
• Creating the correct mypkg directory structure
• Copying and renaming the binary and resources.neu
• Writing the launcher shell script with correct permissions
• Adding a 256×256 app icon
• Creating the .desktop entry for the application menu
• Writing the DEBIAN/control file
• Building the final .deb with dpkg-deb
• A troubleshooting table for common issues

Tested on

• Ubuntu 22.04 / 24.04 (amd64)