docs: improve pr watcher docs
Description
Improves and updates the documenatation for the PR Watcher function
Testing instructions
Check the modified doc from this url https://opencraft.gitlab.io/-/dev/grove/-/jobs/6197949078/artifacts/test/user-guides/automated-pr-sandbox/index.html
Additional context
OpenCraft internal ticket : BB-8541
Edited by Kaustav Banerjee