This MR has a lot of required changes to be an actual port to Qt6, which is why it's big:

* makes QML use versionless imports
* changes contents/ui/ to qml/
* switches QT_MAJOR_VERSION with 6 (since versionless imports won't work on 5)
* switches to declarative type registration [1]
* explains how to create custom QML modules as well
* switches engine.load() to engine.loadFromModule()
* switches actions.{main,left,right} and contextualActions with just actions
* switches Kirigami.Overlaysheet with Kirigami.Dialog
* switches the installed package selection to Qt6/KF6
* switches from BasicListItem to Controls.ItemDelegate/Kirigami.TitleSubtitle et al
* removes mention of the Kirigami Gallery (which wasn't ported until recently and won't be available on distros so soon)

[1] = does not include "connect C++ to QML"

And a few QoL fixes that improve the overall experience:

* removes broken reflinks
* show full code for examples
* update all images to make Kirigami attractive as a product
* adds mx-auto to fix spacing between side-by-side images
* adds the br html tag to space images properly when to the side of text
* adds titles for the code to match the images and to make it easier to scan where things are from
* sets width and height to get reasonable default window sizes
* adds a desktop file (for Wayland window icons...)
* switches from `...` to `// ...` so examples are copy-pasteable without breaking things

This MR is not intended to:

* Replace uses of QApplication for metadata with KAboutData (yet)
* Update FormCard (because it must be done in conjunction with the snippet repo)
* Update the explanation on connecting C++ and QML (because it's useful to know both + it's a big enough step on its own)
* Add a "Porting to Kirigami 6" page
* Remove all broken links (because we can in fact fix those Kirigami API links soon-ish)
* Rewrite the tutorial (we need something *now* that users can reference to learn Kirigami while a new one is being made)

To-do:

- [x] rename helloworld -> kirigami-project
- [x] add missing screenshots
- [x] add missing links

To test this MR:

```bash
git clone https://invent.kde.org/documentation/develop-kde-org.git
cd develop-kde-org
git switch work/thiagosueto/kirigami6
hugo serve
# Open localhost:1313
```