[DOC] Search Pipelines - GA

[macohen]

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 and all versions that are affected.
Search Pipelines are going GA in 2.9 so we want to update all related documentation to remove the experimental tags.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
https://opensearch.org/docs/latest/search-plugins/search-pipelines/index/

cc:@msfroh, @noCharger, @mingshl, @sejli

[noCharger]

Major changes

1. Feature flag - default to true
2. Ad-hoc pipeline
   a. original issue Pipeline specified in a search request (ad hoc execution). OpenSearch#6717
   b. code change [Search Pipelines] Support ad hoc pipelines OpenSearch#7253
   c. example request & response
3. Ignore failure flag
   a. add ignore_failure row to each processor page
   b. add example here https://opensearch.org/docs/latest/search-plugins/search-pipelines/index/ and link it on each processor page
   c. example request
4. Metrics [DOC] Search Pipeline Metrics #4511