Add part for matrix username (#556)

* Add part for matrix username

* Fix broken english + better wording

---------

Co-authored-by: TheFrenchGhosty <47571719+TheFrenchGhosty@users.noreply.github.com>

.github/ISSUE_TEMPLATE/new_instance.yaml

@@ -17,10 +17,6 @@ body:
         - Improving the performance and the stability of your public instance: https://docs.invidious.io/improve-public-instance/
         
         It is highly recommended to follow these tutorials because it will allow the instance to stay stable and performant over the long term.
-      
-        Please consider joining our Matrix room for public instance maintainers by joining our Matrix room: https://matrix.to/#/#invidious:matrix.org
-        then pinging @unixfox, @TheFrenchGhosty and @SamantazFox for asking to be invited to the Matrix room.
-        We discuss troubles managing a public instance, sharing some advices and more.
 
   - type: input
     id: url
@@ -89,6 +85,15 @@ body:
     validations:
       required: false
 
+  - type: input
+    id: matrix-username
+    attributes:
+      label: Your matrix username
+      description: "Please input your Matrix username here to join the private Matrix room for public instances maintainers. We discuss troubles managing a public instance, share some advices and more. Optional but highly recommended."
+      placeholder: "matrixuser"
+    validations:
+      required: false
+
   - type: textarea
     id: more-infos
     attributes: