Initial updates to legacy appfw documentation for Flatpaks

content/architecture/apis-and-services.md

content/architecture/application-structure.md

content/architecture/bundle-spec.md

content/architecture/data_sharing.md

content/concepts/application-bundle-metadata.md

content/concepts/application-entry-points.md

content/concepts/application-framework.md

content/concepts/application-layout.md

content/concepts/applications.md

content/concepts/compositor_security.md

content/concepts/permissions.md

content/concepts/security.md

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