Document bug reporting for unofficial packages

[lengau]

This attempts to redirect users filing reports for unofficial packages to those packages' issue trackers.

Spawned from the conversation at casperdcl#7 (comment)

[mislav]

Hi, thank you for proposing these changes, but after some deliberation I am thinking that it doesn't significantly improve our docs, so I would leave these notes out for now to keep our docs and our issue templates simple.

The language used in these notices assumes that the user always knows if they were using an official package or not, but the problem is that most users don't know that. For example, someone might see our Ubuntu installation instructions and think “oh, they have an Ubuntu package!” and type apt install gh in their system, thus unwittingly obtaining an unofficial package from Snap instead of installing from our own PPA. Others see that we maintain some Linux packages and then go install gh with a package manager of their choice, which could depending on their OS and "sources" configuration result in an unofficial package being installed.

Lastly, we don't want to maintain a list of URLs to issue trackers of other projects. Ideally, the gh binary would include information about how it was built and what is the issue tracker URL, with that information being injected at build time. (Currently, only the build date is injected at build time.) This information would be printed from gh help or gh version. But as far as our online installation docs go, I would keep them as is without further fragmentation of "unofficial" vs. official versions.