[DOC] Add documentation for the Data Prepper startsWith function

[graytaylor0]

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.

This change adds a new function to Data Prepper, startsWith, which is very similar to the contains function (https://opensearch.org/docs/latest/data-prepper/pipelines/contains/), but for checking if a string starts with another string as the prefix

*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/data-prepper#4840