Improve README front door for Bash-only adopters

[codeforester]

Summary

• add a focused “When to reach for Base Bash” section for environments that must remain on Bash
• lead with the five-minute path and examples instead of a broad language-runtime comparison
• move governance, policy, release, support, and community links into a scannable documentation map

Validation

• git diff --check

Fixes #303
Fixes #304