Change Tasks documentation to describe the widget experience

Currently the documentation page for Tasks focuses on the Markdown flow. Since the MVC will focus on the Widget experience this documentation should be changed.

Pages to be updated

• https://docs.gitlab.com/ee/user/tasks.html
• https://docs.gitlab.com/ee/user/permissions.html#project-members-permissions

What should be changed

• Description of feature flags
• Instructions for creating, editing and deleting a task
• Permissions for Work Items

The flash message calling out how to enable the FF should be removed or edited with the rollout of that feature flag.