docs: add private escrow kit guide

[critesjosh]

Summary

• Add a new Private Escrow Kit developer docs section.
• Cover the kit workflow, privacy model, escrow lifecycle phases, generated project structure, testing guidance, and limitations.
• Add the new page vocabulary to docs/docs-words.txt.

Validation

• git diff --cached --check
• git show --stat --oneline --check HEAD
• yarn spellcheck from docs/