feat: Add release notification workflow (#1457)

## Problem

No automated way to notify the Discord community when a new app release is published.

This will play nicely when we have proper release changelogs as well.

## Changes

1. Add code-discord-release.yml workflow triggered on GitHub release publish
2. Use SethCohen/github-releases-to-discord action to post release notes
3. Requires DISCORD_RELEASE_WEBHOOK_URL repo secret

## How did you test this?

I tested the webhook and set the secret, but not this action.

Author: charlesvien

.github/workflows/code-discord-release.yml

@@ -0,0 +1,16 @@
+name: Release PostHog Code to Discord
+
+on:
+  release:
+    types: [published]
+
+# No GITHUB_TOKEN permissions needed -- only calls an external webhook
+permissions: {}
+
+jobs:
+  notify:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: SethCohen/github-releases-to-discord@37afa88c8c9302a9307244b5a0d4e782d528a4b5 # v1.15.1
+        with:
+          webhook_url: ${{ secrets.DISCORD_RELEASE_WEBHOOK_URL }}