docs: sync Verawood dev/operator release notes with wiki updates

[feoh]

Description

Syncs the Verawood developer & operator release notes with the latest content on the Verawood - Operator Release Notes wiki page, which has been updated since these notes were first drafted.

Changes:

• Corrected the Upgrade/upsell widget entry (enabled by default, updated config keys/slot names).
• Added the new "notifications enabled by default" opt-out waffle flag change.
• Documented removal of course-level notification preferences, the is_core preference flag, and the V2 notification preferences config API.
• Expanded the send_email_digest deprecation entry with the new digest scheduling settings.
• Documented new operator-facing notification settings: instance-level preference overrides, the immediate-email buffer window, and ORA reminder notifications.
• Added the "Frontend-base" and "Loading External Scripts" sections, which existed on the wiki but were missing from these notes.

Supporting information

Refs openedx/wg-build-test-release#610

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