Revise README, symlink to main package

[huntie]

Summary

Generally simplify root README, and move in place of main @react-native-community/cli package README so that it is surfaced on npm.

Changes

• Update/simplify wording, add mention of the Community Template repo.
• Remove build, license, PRs welcome badges (all have equivalent CTAs on the GitHub homepage).
• Remove Lean Core references.
• Truncate old versions in compatibility table.
• Delete "Contents" and "Updating the CLI" sections.
• Add permalinks to Docs section.

Test Plan

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Checklist

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