[DOCS] Relocate Watcher Documentation to ES Ref Guide

[jrodewig]

Move Watcher-related documentation from the Stack Docs
to the Elasticsearch Reference Guide.

• Remove content from Stack Overview: Removes Watcher from Stack Overview stack-docs#546
• Add content to the Elasticsearch Guide: [DOCS] Moves Watcher content into Elasticsearch book #47147
• Add redirects: https://github.com/elastic/website-www.elastic.co/issues/4707, Adds redirects for Watcher content docs#1222, Adds more Watcher redirects docs#1247
• Update links from Kibana Guide: [DOCS] Updates links to Watcher content kibana#46636
• Update links from Cloud: https://github.com/elastic/cloud/pull/41125

[elasticmachine]

Pinging @elastic/es-docs

[elasticmachine]

Pinging @elastic/es-core-features