[chore] Update legend to demo (#2505)

* fix: update `legend.lnbits.com` to `demo.lnbits.com`
* Update docs/guide/admin_ui.md
* fix: docker image name
* fix: donation links

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Co-authored-by: dni ⚡ <office@dnilabs.com>

docs/guide/faq.md

@@ -84,7 +84,7 @@ allow-self-payment=1
 <details><summary>Wallet-URL deleted, are my funds safu ?</summary>
     <ul>
       <li>
-        <details><summary>Wallet on demo server legend.lnbits</summary>
+        <details><summary>Wallet on demo server demo.lnbits.com</summary>
         <p>Always save a copy of your wallet-URL, Export2phone-QR or LNDhub for your own wallets in a safe place. LNbits CANNOT help you to recover them when lost.</p>
         </details>
       </li>