Documentation: Add a document to explain how to connect to SSHServer

Documentation/SSHServer.md

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+# Connecting to Serenity's SSH Server
+
+Serenity's SSH server currently supports only a subset of standard SSH features.
+
+## Authentication
+
+The server only supports the `publickey` authentication method.
+
+-   Your **client** must have a private key.
+-   The **server** must have access to the corresponding public key.
+
+> NOTE: Only **ED25519** keys are supported. Other key types (RSA, ECDSA, etc.) will not work.
+
+### Generating a Key Pair
+
+To generate a compatible key pair, run the following command on your host machine:
+
+```bash
+ssh-keygen -t ed25519 -f ~/.ssh/serenity_ed25519
+```
+
+### Installing the public key in SerenityOS
+
+To allow SSH access, you need to install your public key into the SerenityOS image so the SSH server can authenticate your client.
+
+The SSH server looks for authorized keys in `/home/anon/.config/ssh/authorized_keys`.
+The simplest way to do it is to add these lines to `sync-local.sh`:
+
+```bash
+mkdir -p mnt/home/anon/.config/ssh/
+cat path/to/serenity_ed25519.pub >> mnt/home/anon/.config/ssh/authorized_keys
+```