Polish README for launch visitors

[duriantaco]

Summary

• Move install guidance above the long usage reference so new visitors can try jonq immediately.
• Tighten the opening pitch around inspecting unfamiliar JSON, readable SQL-like queries, and generated jq.
• Surface typo repair earlier in the README, before the detailed feature reference.

Validation

• python -m pre_commit run --all-files
• LC_ALL=C LANG=C python -m sphinx -b html docs/source /tmp/jonq-docs-readme-polish
• Manual typo-repair check: python -m jonq tests/json_test_files/simple.json "select nme"