Download Mu: add mention to Debian/Ubuntu repositories

[cpina]

Debian and Ubuntu have mu-editor in their repositories. I think that it would be useful to mention this in https://codewith.mu/en/download.

It's often the easier way to install software in Debian/Ubuntu. Centralised, integrated with the system's update, etc. (even though it might be older and less control from mu-editor team, I know).

[prcutler]

Is this something we want to do? If we do it for Ubuntu and Debian, do we need to do it for Fedora or other distros?

Since the download page says that AppImage is still experimental, what if we added a sentence such as:

Mu may also be available via your Linux distribution's package manager.

The risk being, as called out above, that it might be older.

If there is interest, I can work on a PR