add documentation for watchdog 3rd party monitoring

[HugoPct]

What does this PR do? What is the motivation?

add documentation for watchdog 3rd party monitoring, new feature that we are soon going to release.

Merge instructions

Below items to be tackled before the release of this docs (on me)

• Roll out the feature
• Roll out the blog post
• Edit the blog post URL in the FURTHER READING section of this docs page
• Get a legal review

Merge readiness:

• Ready for merge

Merge queue is enabled in this repo. To have it automatically merged after it receives the required reviews, create the PR (from a branch that follows the <yourname>/description naming convention) and then add the following PR comment:

/merge

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/HugoPct/add-3p-docs/watchdog/faulty_cloud_saas_api_detection

[aliciascott]

Hi @HugoPct general comment on this before we start the review:

1. Is this a new doc/different from the Faulty Detection here? https://docs.datadoghq.com/watchdog/faulty_deployment_detection/
2. If so it should also probably be added to the menu file.
3. The Further Reading isn't showing up in the staging preview, because of this incorrectly formatted URL "https://www.datadoghq.com/blog/<URL FROM THE BLOGPOST>/". We do have a process to add blog posts to docs pages after they are published so you can remove this section from further reading for now.

Let us know on the above once you get this added to the menu/left Nav, etc and then we can start a review, thanks!
I've added a Work in Progress label for now but it can be removed once it's ready for review.

[HugoPct]

Hi @aliciascott
1.2. ok added to the menu file
3. done

[buraizu]

Thanks, left some suggestions! Let me know if you have any questions.