Add Matrix group

Signed-off-by: Lin Jian <me@linj.tech>

src/content/docs/en/index.mdx

@@ -10,8 +10,12 @@ hero:
         - text: 'Meetup #2 Attendee Guide'
           link: /en/meetup-2-guide
           icon: right-arrow
-        - text: Join the event group
+        - text: Join the Telegram event group
           link: https://t.me/nixcnmeetup
           icon: external
           variant: minimal
+        - text: Join the Matrix event group
+          link: https://matrix.to/#/#nix-cn-meetup:rebmit.moe
+          icon: external
+          variant: minimal
 ---

src/content/docs/index.mdx

@@ -10,8 +10,12 @@ hero:
         - text: 'Meetup #2 参会指南'
           link: /meetup-2-guide
           icon: right-arrow
-        - text: 加入活动群组
+        - text: 加入 Telegram 活动群组
           link: https://t.me/nixcnmeetup
           icon: external
           variant: minimal
+        - text: 加入 Matrix 活动群组
+          link: https://matrix.to/#/#nix-cn-meetup:rebmit.moe
+          icon: external
+          variant: minimal
 ---