flatpak/index.md: add related topics

[kbdharun]

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• I am not the sole author or employee of a company who created the topic or collection I'm changing.

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection
• Something that does not neatly fit into the binary options above

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288, size <= 75 kB)
• Content (and my changes are in index.md)

This PR links flathub and linux topic to the current flatpak topic as Flatpak is a packaging format for packaging and distributing apps for Linux and Flathub is the defacto remote for publishing Flatpaks.

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[ipc103]

👍 ❤️