guides: Add flatpak signature guide

This commit adds a user guide explaining how to create flatpaks signed with the Ed25519 algorithm, and how to verify such flatpaks.

content/guides/flatpak.md

@@ -6,12 +6,19 @@ title = "Application verification using Flatpak"
 +++
 
 Starting with Apertis v2022dev2, [Flatpak](https://flatpak.org) includes the
-ability to distribute [application bundles](/glossary/#application-bundle)
+ability to distribute [application bundles]( {{< ref "glossary.md#application-bundle" >}} )
 verified with [ed25519](https://ed25519.cr.yp.to/) signatures.
 
-This signature system rely on ostree's library functions. Therefore, the key
+{{% notice warning %}}
+The current implementation is a technology preview and it is expected to stabilize
+during the release cycles leading to the v2022 stable release. The prototype is
+already available in Apertis as documented here, but it may be subject to potentially
+incompatible changes during the [upstream review process](https://github.com/flatpak/flatpak/pull/4170).
+{{% /notice %}}
+
+This signature system relies on OSTree's library functions. Therefore, the key
 generation and storage process is identical to what is described in the
-[System updates and rollback](/designs/system-updates-and-rollback/#verified-updates)
+[System updates and rollback]( {{< ref "system-updates-and-rollback.md#verified-updates" >}} )
 design document.
 
 Flatpak application signatures occur on several levels:
@@ -27,7 +34,7 @@ verified. Similarly, it is not possible to sign flatpak applications using GPG.
 
 The simplest way to create a signed flatpak is to use `flatpak-builder` with
 the `--sign=<SECRETKEY>` command-line argument, where `<SECRETKEY>` is the
-base64-encoded secret Ed25519 key. This ensures the ostree commit and summary
+base64-encoded secret Ed25519 key. This ensures the OSTree commit and summary
 are properly signed.
 
 For more advanced usage, the same command-line option can also be used with the