[Docs migration] Cleans up investigation tools files - pt 2 (#565)

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:

-
[Notes](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/565/solutions/security/investigate/notes)
-
[Cases](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/565/solutions/security/investigate/cases)
- [Cases
requirements](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/565/solutions/security/investigate/cases-requirements)
- [Open and manage
cases](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/565/solutions/security/investigate/open-manage-cases)
- [Configure case
settings](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/565/solutions/security/investigate/configure-case-settings)

---------

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