[DOC] Anomaly Detection Feature Direction documentation

[amitgalitz]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.
I want to add more to the Anomaly Detection documentation. This would be on the main page for the AD plugin where we highlight the steps to create a detector. I will add another paragraph to explain our new feature in 2.19 that includes deciding on a general direction for each feature, whether users want anomalies to be detected only if the actual values goes either above or below the expected value.

code issue: opensearch-project/anomaly-detection#616

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
2.19
What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

[amitgalitz]

Doc PR: #9137