[repo.xml]: Add how to on customizing app icon

[joewiz]

Describe the problem

The Package Repository article should describe how app icons work and how to supply one's own icon for an app. While not explicitly part of repo.xml or expath-pkg.xml, the icon facility is part of a package's metadata. It represents the app in apps like the public repo and the dashboard package manager. Its naming convention, supported formats, and any other important guidelines (resolution? dimensions? aspect ratio?) should be documented. The location of the default eXist icon should also be noted.

Context

On the e-editiones Slack community channel, @arojascastro asked:

do you know how to change the logo that appears in the eXist Launcher?

@duncdrum responded:

add your own icon.svg or icon.png at the root of your application repo.

you ll probably have to restart to see the changes take effect

@djbpitt suggested that this should be documented:

For what it’s worth: When I needed, a couple of years ago, to change the logo in the dashboard) for an app I was developing, I couldn’t find information about how to do that in the eXist-db documentation. I finally figured it out by copying someone else’s code (thanks, Joe!), but if it’s still missing from the documentation, it would be a small but welcome addition.

Screenshots

Accompanying illustration to @arojascastro's post:

Please provide the following

• exist-db version: 5.2.0
• documentation version: 5.2.0