docs: 📝 Add dynamic avatar guide

apps/docs/docs/guides/multi-avatars.mdx

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+# Dynamic avatars
+
+You can decide to change the host avatar dynamically so that your bot can display multiple avatars. Here is a simple example:
+
+<img
+  src="/img/guides/multi-avatars/demo.gif"
+  width="800"
+  alt="Multiple avatars demo"
+/>
+
+For this, head over to the "Theme" tab and set the host avatar to a variable URL:
+
+<img
+  src="/img/guides/multi-avatars/bot-variable.png"
+  width="500"
+  alt="Variable URL bot"
+/>
+
+Then you can set this variable to any URL in your flow so that the host avatar changes dynamically.
+
+<img
+  src="/img/guides/multi-avatars/flow-example.png"
+  width="800"
+  alt="Multiple avatars flow example"
+/>
+
+The first block defines all the avatars that can be displayed. The second block starts by setting the avatar to the wizard avatar. The final block starts by setting the swordsmith avatar.