Doc: Outdated export/import instructions

(The list below is not comprehensive)

• https://doc.qt.io/qtdesignstudio/figmaqtbridge.html: "export designs from Figma to .metadata format"
• https://doc.qt.io/qtdesignstudio/studio-importing-2d.html: "Select Exported Assets (.metadata)* in the dropdown menu to filter .metadata files."

• https://doc.qt.io/qtdesignstudio/qtbridge-figma-using.html: "Before importing the design into Qt Design Studio, you have to manually extract the archive. Then you can import the .metainfo into a project in Qt Design Studio"

.metainfo never existed AFAIK, the format has changed from .metadata to .qtbridge, and it is no longer necessary to manually extract the .qtbridge file.