Update workflow to allow more rapid changing of live release notes

[stevejalim]

At the moment if an edit/fix/change to an already-published release note is needed, there can be a wait of several minutes before the changes are reflected in Bedrock production. This is because of one/some/all of the following:

• release notes are synced every 10 mins, as a pull into bedrock's DB from the release-notes repo
• bedrock updates the shared sqlite database every 5 mins
• live pods look for a new DB to pull down every 5 mins
• the CDN's TTL is set to ~5 (?) minutes.

We could address this to reduce it to at worst 5+5 mins by:

• changing the release-notes update step from pull to push (via a webhook triggered from nucleus or from the intermediary GH repo)
• triggering a CDN-level invalidation of any specific release-notes URLs which have changed (which we could also include the the webhook payload)

Thoughts and suggestions welcome here and we'll edit this description to fit with what we decide to do