Watcher mentions often missing "not available on serverless" context

We have lots of mentions of watcher throughout the docs. for example:

https://www.elastic.co/docs/explore-analyze/report-and-share/automating-report-generation#use-watcher

These mentions often don't specify that Watcher is unavailable in Serverless (bad).

## Action

Audit docs for mention of watcher

On pages with serverless tags, add a tag on the watcher content to specify that watcher is not available on serverless