Kibana 8.19.0 release notes and what's new

[florent-leborgne]

• Generate and edit release notes for 8.19.0
• Update what's new page --> TBD if we'll publish this page or already adopt the 9.0+ model where we instead link to the release blog.

[florent-leborgne]

@nastasha-solomon @wajihaparvez FYI I tested generating these and many PRs were mislabeled so I've asked devs to fix it. Will try again in a few days. There will be a long list for us to edit :D