Improve Airflow integration documentation

[ritalwar]

Lately, we've noticed a number of cases where Airflow metrics weren't getting stored in Elasticsearch. Many times, this boils down to configuration problems. To prevent such issues in the future and enhance user experience, let's review our documentation for any gaps and update it accordingly.

Reference issues:
#9748
https://github.com/elastic/sdh-beats/issues/4680