Recommend JSON for flatpak manifest instead of YML

[danirabbit]

Problem

YML feels easier for the simplest case but for more complex cases tools like External Data Checker require using JSON. It's a pain to tell a developer to use YML for their manifest until their app wants to add a new feature with a dependency outside our platform and now they have to convert their manifest to JSON for the best experience

Proposal

We should just recommend JSON from the beginning

Prior Art (Optional)

No response