Establish official document for relaying specs to the SDET team

This task is about establishing a document in which the Consensus writes the specification for features that SDETs are testing and that Consensus is mostly responsible for (eg the forecast horizon, especially how it is used in Plutus-laden transactions validity intervals).

This document can be either located at
- ouroboros-consensus/docs/specsForSdetTeam.md, or at
- cardano-node-tests/docs/specsForSdetTeam.md (we need to contact the SDET team to confirm this is a suitable place for this).

We prefer this document living in a git repository because we expect we’ll be having contributors from both Consensus and the SDET team, and so the PR process will be very valuable.

# Acceptance criteria
Establish either:

- ouroboros-consensus/docs/specsForSdetTeam.md, or
- cardano-node-tests/docs/specsForSdetTeam.md

and seed it with at least a couple features' specs (to be identified during this task).