(Java) Document P&WS service functionality as on 2024/04/19

Description

Document WHY we do have P&WS service and HOW to use it.

Acceptance criteria

• We have at least /docs/index.md doc (and optional additional docs there)
• at least we have the Introduction and Basics sections and some words about /projects CRUD API

Testing scenarios

• docs exist and readable

Technical notes

• The content is structured similar to what we have for Entitlements or for Storage, etc.