Flight tasks docs needs a refresh/check

[hamishwillee]

The Flight tasks doc has had only limited updates since PX4 v1.11 (we're now after PX4 v1.15 at time of writing).

1. Needs to be reviewed by expert
2. For maintenance would be good to have an example task that can be referred to in-tree and gets updated with the real tasks. That way if the architecture changes, the required changes can be inferred from the test task. Ideally created by engineering?

This replaces #3421 which has a lot of "support" discussion.