[Docs migration] Cleans up detection and alerts files (#566)

Addresses part of #559 by
doing the following:
- Removes raw migrated Serverless files that are no longer being used.
- Adds anchors where missing and updates anchors that start with an
underscore.
- Makes minor inline adjustments to the files that were labeled as
"lift-and-shift" where appropriate.
 
Affected pages:
- [About building block
rules](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/about-building-block-rules)
- [About detection
rules](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/about-detection-rules)
- [Add detection alerts to
cases](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/add-detection-alerts-to-cases)
- [Create a detection
rule](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/create-detection-rule)
- [Detections
requirements](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/detections-requirements)
- [Install and manage Elastic prebuilt
rules](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/install-manage-elastic-prebuilt-rules)
- [Launch Timeline from investigation
guides](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/launch-timeline-from-investigation-guides)
- [Manage detection
alerts](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/manage-detection-alerts)
- [Manage detection
rules](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/manage-detection-rules)
- [MITRE ATT&CK®
coverage](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/mitre-attandckr-coverage)
- [Using logsdb index mode with Elastic
Security](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/using-logsdb-index-mode-with-elastic-security)
- [View detection alert
details](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/view-detection-alert-details)
- [Visualize detection
alerts](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/visualize-detection-alerts)
- [Alert
suppression](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/suppress-detection-alerts)
- [Reduce alerts and
notifications](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/reduce-notifications-alerts)
- [Query alert
indices](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/query-alert-indices)
- [Tune detection
rules](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/566/solutions/security/detect-and-alert/tune-detection-rules)

---------

Co-authored-by: Benjamin Ironside Goldstein <[email protected]>

Author: 2 people