Introduce onboarding popover for the decision log button

What does this MR do?

This MR adds a one-time popover that introduces the decision log to a user the first time they see the decision log button in the work item header. The popover explains the purpose of the decision log, notes that decisions can be recorded by hand, and links to the documentation. The user dismisses it with the close button, or by opening the decision log, and it does not come back, because the dismissal is stored against the user.

This MR also removes the gate that only showed the decision log button on work items with AI planning turned on. The decision log does not need a workplan, so the button now appears on every work item. It stays behind the decision_log feature flag, which is off by default.

This MR also adds a Decision log section to the workplan documentation. No user documentation for the decision log existed, and the popover needs somewhere to link to.

Screenshots

How to test

1. Turn on the decision_log feature flag.
2. Open any work item as a user who has not seen the popover. The popover appears next to the decision log button in the header.
3. Select the close button, or select the decision log button. Reload the page. The popover does not come back.

References

Closes #627677 (closed)