[Docs migration] Cleans up getting started files - Pt. 2

[nastasha-solomon]

Follow-up to #497, which was recently rescoped.

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:

• Elastic Security requirements

[natasha-moore-elastic]

A couple of minor suggestions, otherwise LGTM!