merge collaborative txn privacy chapter #60

move the chapter from #7 into docs/collaborative_txn_privacy.md below the stub's frontmatter, changing its type from Stub and adding a description, and move its docs/index.md entry out of "In Review" with that description. also bring .typos.toml from #7 (it allows the Mermaid class aci) and its README sentence, with the link pointing at docs/collaborative_txn_privacy.md.