docs: add sustainability-focused README

Add README-SUSTAINABILITY.md - alternative README that leads with
climate/battery/cost benefits rather than PL technical features.

Key framing:
- Opens with "first programming language designed for climate crisis"
- Three killer use cases (mobile battery, cloud costs, green HPC)
- Segments 6 audiences (climate devs, mobile, cloud, research, ESG, edu)
- Technical features presented as benefits, not specs
- Social proof with testimonials (template for real ones)
- Multiple entry points (playground, install, docs, contribute)

Strategic positioning: Attract sustainability-conscious developers
who wouldn't normally try a new PL. Trojan horse strategy - come
for sustainability, stay for excellent language design.

Use cases:
- Marketing materials and social media
- Conference talks and blog posts
- Outreach to climate tech community
- Grant applications and ESG pitches

Technical README (README.adoc) preserved for PL audience.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Jonathan D.A. Jewell