[Internal][Workflows][9.3 & Serverless][Phase 1]: Introductory material and foundational concepts

[nastasha-solomon]

Areas to cover:

• What are workflows?
• How do they work?
• Who should use them?
• How do I create my first workflow? (Will need to also provide guidance for turning on the workflow feature.)

[nastasha-solomon]

This content is (tentatively) going in the following part of the workflow TOC:

• Workflows (What are they? How do they work? Who should use them?)
  • Get started (basic tutorial)
  • Use cases (ES, Obs, and Sec)

[nastasha-solomon]

Kibana advanced setting docs are being updated via elastic/kibana#249953.

[nastasha-solomon]

Overview and tutorial added in #4713.