Show a welcome screen on new installs and upgrades

Two things I (and many others, no doubt) have noticed: New users very often don't realise there's a manual, or at least think they don't need it, and current users don't always realise they are on a new version with new features, or don't know where the release announcement is.

So my proposal is to add a pop-up screen when darktable is launched for the first time after being installed or upgraded, that has a section for new users that encourages them to look at the FAQ and manual, and another section that at a minimum links to the release notes, but maybe even includes the headlines.

This should only be shown once, unless the user specifically ticks the box to have it shown next launch, but there should be a button in the about dialog to get to it as well.

So it would look something like this:

Welcome to darktable X.Y.Z

New users

To help you get started, make sure to look at the FAQ and read the overview section of the manual.

• FAQ
• Manual

New in this version

• Release notes

[ ] Show this screen on next launch

Start by reviewing darktable’s startup flow and the existing About dialog, since the issue requires a first-launch or post-upgrade screen plus an About-dialog entry point. Confirm how the FAQ, manual, and release notes are linked. Done means the screen appears only as specified, supports the checkbox behavior, and provides the requested links and version information.