Cleanup the following sections: - [x] Get started with Elastic Security: https://github.com/elastic/docs-content/pull/497 - [x] Detections and alerts: https://github.com/elastic/docs-content/pull/566 and https://github.com/elastic/docs-content/pull/789 - [x] Investigation tools : https://github.com/elastic/docs-content/pull/498 and https://github.com/elastic/docs-content/pull/565 Cleaning tasks consist of: - Removing raw Serverless files (text only for now) that are no longer being used. - Adding anchors where missing and updating anchors that start with an underscore. - Making minor inline adjustments to the files where appropriate. - Add section-level [lifecyle and versioning](https://elastic.github.io/docs-builder/versions/) information at the appropriate levels. (Page level tags will be bulk-added in a later phase.)