Doc/Michelson: document the TZT Michelson extension for writing unit tests

This doc-only MR extends the Michelson documentation with a description of the TZT format.

Related work:

• TZT reference test suite: https://gitlab.com/tezos/tzt-reference-test-suite/
• TZT runners: (for octez-client: %Support for Michelson unit tests in TZT format, for Michelson-in-Rust: !10269 (merged), for Mi-cho-coq: nomadic-labs/mi-cho-coq!67)