docs: align Flatpak build notes

[pitill0]

Summary

Align the Flatpak documentation with the current dependency and AppStream build flow.

Changes

• Remove the obsolete appstream-compose: false note.
• Document that Python dependencies are installed from the checked-in python3-requirements.json module.
• Clarify that Flatpak builds use pinned source URLs and checksums instead of resolving application dependencies through pip.
• Confirm that AppStream metadata is installed and validated as part of the Flatpak build.