Extend documentation for states of DAGs & tasks and update trigger rules docs

[mnojek]

The story behind this change:
This change is somehow inspired by the AIP-47 that is still in the discussion phase. In this new design of system tests, we need to make use of Trigger Rules for handling teardown tasks (e.g. for cleaning resources required by recently executed the system test). The main concern is that when we use teardown task with trigger rule all_done (to make sure that it is executed even if something's gone wrong in the middle of the test), the whole test (which is just a DAG) will take the result from this particular teardown task and we can lose the information about some failing task in the middle. This is not expected workflow for the tests, because we want the test to fail if any step (task) failed. The reason why the whole test gets the status of a teardown task and not signalizes that anything failed in the middle is that Airflow works like this - the DAG Run status is determined by the status of the "leaf nodes" (the tasks that do not have any children). Since the teardown task is the leaf node, the whole test gets the same status (which is almost always success).
That's why we need to have another watcher task with trigger rule set to one_failed that is a downstream task for any other task in the test (DAG). Thanks to this, it will be triggered if any of the task in the DAG failed and thus its status will be propagated to the DAG Run (because it is a leaf node).

By doing the research in the documentation and code, I found it very difficult to find the information how trigger rules work in the details and that's why I thought that it would be good to extend the documentation for them. Since I already spent some time to understand it deeply, I also took the effort and prepared this PR. It's not big, but it took me several hours to prepare these statements. I am not sure if all the statements are correct, so please read it carefully and correct me if I'm wrong and I will edit the PR.

On the other hand, I would like to also start a discussion about the trigger rules. To me they seemed simple at first, but the more time I spent with them I figured out that they introduce a lot of complications to the task execution. I hope that this PR will make it easier to understand how they work. If you have any ideas how we can make them even better, I am glad to discuss it.

[github-actions]

The PR is likely ready to be merged. No tests are needed as no important environment files, nor python files were modified by it. However, committers might decide that full test matrix is needed and add the 'full tests needed' label. Then you should rebase it to the latest main or amend the last commit of the PR, and push it with --force-with-lease.