Documentation: Ubuntu *.deb based install instruction (and other OS specific install instructions)

### Feature summary

Documentation needs explain *.deb, *.rpm install instruction.

### What problem are you trying to solve?

AppImage based installation on Ubuntu is complex.
Issues:
1) Automatic updates, problems with permissions
2) Desktop icon is not GH App
3) At some point it started to be extremely slow and eat 90%+ CPU

### Proposed solution

The *.deb installation have no any of these problems.

It's hard to find which apt source should be used, so instruction would help.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

Start by locating the installation documentation and the existing AppImage instructions. Add clear Ubuntu *.deb guidance, including which apt source to use, and cover *.rpm or other OS-specific installation instructions where applicable; done means users can find and follow the supported package-install steps.

索引モデルが issue の本文から書いたものです。