[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

---------

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

docs/devs/api.md

@@ -7,4 +7,4 @@ nav_order: 3
 
 # API reference
 
-[Swagger Docs](https://legend.lnbits.com/docs)
+[Swagger Docs](https://demo.lnbits.com/docs)

docs/devs/swagger.html

@@ -16,7 +16,7 @@
       window.onload = function () {
         // Begin Swagger UI call region
         const ui = SwaggerUIBundle({
-          url: 'https://legend.lnbits.com/openapi.json', //Location of Open API spec in the repo
+          url: 'https://demo.lnbits.com/openapi.json', //Location of Open API spec in the repo
           dom_id: '#swagger-ui',
           deepLinking: true,
           presets: [

docs/devs/websockets.md

@@ -9,7 +9,7 @@ nav_order: 2
 
 `websockets` are a great way to add a two way instant data channel between server and client.
 
-LNbits has a useful in built websocket tool. With a websocket client connect to (obv change `somespecificid`) `wss://legend.lnbits.com/api/v1/ws/somespecificid` (you can use an online websocket tester). Now make a get to `https://legend.lnbits.com/api/v1/ws/somespecificid/somedata`. You can send data to that websocket by using `from lnbits.core.services import websocketUpdater` and the function `websocketUpdater("somespecificid", "somdata")`.
+LNbits has a useful in built websocket tool. With a websocket client connect to (obv change `somespecificid`) `wss://demo.lnbits.com/api/v1/ws/somespecificid` (you can use an online websocket tester). Now make a get to `https://demo.lnbits.com/api/v1/ws/somespecificid/somedata`. You can send data to that websocket by using `from lnbits.core.services import websocketUpdater` and the function `websocketUpdater("somespecificid", "somdata")`.
 
 Example vue-js function for listening to the websocket: