Problem: contributors can't tell which spec format to use

[zolotokrylin]

Problem

docs/CONTRIBUTING.md documents this Spec format:

---
goal: <link to Goal issue>
---

# Feature Name

## Overview

What this Goal enables for users.

But the format already used across multiple repos is different — Objective, Key Results, Users, and Core Concepts (with user-facing methods per concept):

• https://github.com/holdex/bridged-iq/blob/main/docs/specs/ar-module.md
• https://github.com/holdex/partnerships/blob/main/docs/specs/partnership-program.md
• https://github.com/holdex/Curia-Regis-app/blob/main/docs/specs/initial-bug-fixing.md

Contributors writing a new spec can't tell which format is actually expected.

Solution

Update the Format section of docs/CONTRIBUTING.md to document the Objective / Key Results / Users / Core Concepts structure, with a short example, so the docs match the norm already in use.