[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>

lnbits/core/templates/core/index.html

@@ -338,7 +338,7 @@
           outline
           color="grey"
           type="a"
-          href="https://legend.lnbits.com/paywall/GAqKguK5S8f6w5VNjS9DfK"
+          href="https://demo.lnbits.com/lnurlp/link/fH59GD"
           target="_blank"
           rel="noopener noreferrer"
           :label="$t('donate')"

lnbits/settings.py

@@ -157,7 +157,7 @@ class FakeWalletFundingSource(LNbitsSettings):
 
 
 class LNbitsFundingSource(LNbitsSettings):
-    lnbits_endpoint: str = Field(default="https://legend.lnbits.com")
+    lnbits_endpoint: str = Field(default="https://demo.lnbits.com")
     lnbits_key: Optional[str] = Field(default=None)
     lnbits_admin_key: Optional[str] = Field(default=None)
     lnbits_invoice_key: Optional[str] = Field(default=None)