Add task.allowAutomaticTasks setting documentation

[aritradhabal]

Adds a "Controlling automatic task execution" section to the Tasks documentation covering the task.allowAutomaticTasks setting:

• Documents all three values: off (default), on, and auto
• Notes the default change from on to off with a link to the v1.109 release notes
• Includes a JSON settings example and mentions the Manage Automatic Tasks command

This setting was previously only mentioned in the release notes and had no dedicated documentation.

Fixes #9847

[aritradhabal]

@microsoft-github-policy-service agree