chore(kb): start article about the signal messaging app

knowledge base/signal.md

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+# Signal
+
+Open source application for secure messaging.
+
+Available for [Android][android app], [iOS][iOS app], Linux, Mac OS X, and Windows.
+
+1. [TL;DR](#tldr)
+1. [Further readings](#further-readings)
+   1. [Sources](#sources)
+
+## TL;DR
+
+Conversations are end-to-end encrypted.
+
+Each and every one-to-one chat has its own unique _safety number_.<br/>
+It allows verifying one is communicating with the correct contact.
+
+When a safety number is marked as verified, a checkmark will appear in the chat header by that contact's name.<br/>
+It will remain like that unless the safety number changes or one manually changes the verification status.
+
+Signal notifies the user whenever a safety number has changed.
+
+When the safety number changes, it's as if one changed the locks and keys for the conversation.<br/>
+Messages sent _before_ the safety number changed use the old locks, and as such they will **not** be deliverable.
+
+SMS/MMS messaging _was_ supported in Android systems, but it has been removed
+[since october 2022][removing sms support from signal android].
+
+<!-- Uncomment if used
+<details>
+  <summary>Setup</summary>
+
+```sh
+```
+
+</details>
+-->
+
+<!-- Uncomment if used
+<details>
+  <summary>Usage</summary>
+
+```sh
+```
+
+</details>
+-->
+
+<!-- Uncomment if used
+<details>
+  <summary>Real world use cases</summary>
+
+```sh
+```
+
+</details>
+-->
+
+## Further readings
+
+- [Website]
+- [Codebase]
+
+### Sources
+
+- [Help]
+- [Removing SMS support from Signal Android]
+
+<!--
+  Reference
+  ═╬═Time══
+  -->
+
+<!-- In-article sections -->
+<!-- Knowledge base -->
+<!-- Files -->
+<!-- Upstream -->
+[Android app]: https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms
+[Codebase]: https://github.com/signalapp
+[Help]: https://support.signal.org/hc/en-us
+[iOS app]: https://apps.apple.com/app/id874139669
+[Removing SMS support from Signal Android]: https://signal.org/blog/sms-removal-android/
+[Website]: https://signal.org/
+
+<!-- Others -->