Document FOCUS conformance

[flanakin]

🛠️ Description

Document the conformance gaps in Cost Management FOCUS 1.0 exports compared to the FOCUS 1.0 specification.

📋 Checklist

🙋‍♀️ Do any of the following that apply?

• [ ] 🚨 This is a breaking change.
• [ ] 🤏 The change is less than 20 lines of code.

📑 Did you update docs/changelog.md?

• [ ] ✅ Updated changelog (required for dev PRs)
• [ ] ➡️ Will add log in a future PR (feature branch PRs only)
• [ ] ❎ Log not needed (small/internal change)

📖 Did you update documentation?

• [x] ✅ Public docs in docs (required for dev)
• [ ] ✅ Internal dev docs in src (required for dev)
• [ ] ➡️ Will add docs in a future PR (feature branch PRs only)
• [ ] ❎ Docs not needed (small/internal change)

[arthurclares]

Removed "needs review" tag as PR still in draft stage. Please, add it back once ready to review.

[flanakin]

Reviewed offline