[QDoc] Make Kirigami API more presentable

This is just to improve the presentation of the docs, it doesn't fix everything or touch documentation contents. I want this for documentation/develop-kde-org!570 (merged)

This MR fixes a bunch of layout, link, and minor issues in the Kirigami API.

Currently:

• https://api-staging.kde.org/kirigami-index.html lacks a description and there's an extra Kirigami Delegates link
• https://api-staging.kde.org/org-kde-kirigami-qmlmodule.html and similar are lacking brief descriptions, creating silly gaps
• https://api-staging.kde.org/qml-org-kde-kirigami-action.html#autoExclusive-prop and similar get extra asterisks because QML files shouldn't use asterisks for docs
• https://api-staging.kde.org/qml-org-kde-kirigami-dialogs-searchdialog.html and https://api-staging.kde.org/qml-org-kde-kirigami-contextualhelpbutton.html are missing images
• https://api-staging.kde.org/qml-org-kde-kirigami-platform-theme.html and similar are missing descriptions in the QML types but not in the C++ classes and vice-versa
• https://api-staging.kde.org/qml-org-kde-kirigami-wheelhandler.html and similar are missing examples

This MR:

• add an actual description to the index
• missing examples
• missing images
• unnecessary extra link to Kirigami Delegates
• in the main module pages, make all brief descriptions show up properly
• make links more accessible (https://develop.kde.org/docs/contribute/style/#descriptive-links)
• remove extraneous asterisks that shouldn't be in QML files
• add line spacings where they might improve readability