pages/Welcome: Improve about pages using Kirigami Addons and adding "About KDE"
This gives us consistency with other modern KDE apps, and allows us to also include "About KDE" too. This commit also ensures that the correct icons are used for each action and as the program's logo.
| - | Welcome | About Welcome Center | About KDE |
|---|---|---|---|
| Before |  |
 |
- |
| After |  |
 |
 |
N.B. Non-symbolic icons are used as elsewhere; Kirigami Addons doesn't need to be added to .kde-ci.yml as it was never removed previously when porting from Banner to InlineMessage in 0debce01