fix: [META] - CLA Guidance improvement

[SillyZir]

Summary

Rewrote CLA.md to include clear explanations of what the CLA is, why contributors must sign it, and step-by-step instructions on how to sign it — addressing the lack of guidance reported in #5333.

Fixes #5333

Problem

Contributors interacting with gnokey encountered a "user did not sign CLA" error with no context — no explanation of what a CLA is, why it's required, or how to actually sign it. The existing CLA.md was a dense legal document with no onboarding guidance, and the gnoweb render offered no additional clarity or trust signals.

Solution

Restructured CLA.md by adding three new sections — "What is the CLA?", "Why must contributors sign it?", and "How to sign the CLA" — above the existing legal text. These sections provide a plain-language overview of the agreement, explain the legal rationale (licensing rights, IP confirmation), and walk contributors through the signing flow. The original legal terms were preserved and reformatted for readability.

Impact

New and existing contributors will no longer hit a dead-end error without recourse. The improved CLA.md reduces friction in the contribution onboarding process and builds trust by making the legal requirements transparent and actionable.

[Gno2D2]

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[davd-gzl]

Related: #5325, #5331

[moul]

@alexiscolin can you check with legal?