docs: link the community as stacksjs.com/discord

Replaces every https://discord.gg/stacksjs link with the stacksjs.com/discord
vanity URL, which the stacksjs.com gateway now 301s to the invite. The invite
code lives in one place and can be rotated without touching the docs again.

Author: chrisbbreuer

README.md

@@ -90,7 +90,7 @@ For help, discussion about best practices, or any other conversation that would
 
 For casual chit-chat with others using this package:
 
-[Join the Stacks Discord Server](https://discord.gg/stacksjs)
+[Join the Stacks Discord Server](https://stacksjs.com/discord)
 
 ## Postcardware