Adds external link to ack_watch API

[kosabogi]

DO NOT MERGE BEFORE elastic/docs-content#1768

This PR adds an external link to the Acknowledge a watch API.

[github-actions]

Following you can find the validation results for the API you have changed.

API	Status	Request	Response
watcher.ack_watch	🟢	1/1	1/1

You can validate this API yourself by using the make validate target.

[github-actions]

The backport to 9.0 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-9.0 9.0
# Navigate to the new working tree
cd .worktrees/backport-9.0
# Create a new branch
git switch --create backport-4607-to-9.0
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 92f69aab64ff82e3327985e64454005eb3641209
# Push it to GitHub
git push --set-upstream origin backport-4607-to-9.0
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-9.0

Then, create a pull request where the base branch is 9.0 and the compare/head branch is backport-4607-to-9.0.