chore: update install and release instructions

[zenorocha]

Summary by cubic

Standardized and simplified CLI install instructions across the README and GitHub release notes to make setup clearer and consistent.

• Refactors
  • Made cURL the primary install path.
  • Added npm install -g resend-cli option.
  • Clarified Homebrew support as macOS/Linux and kept PowerShell for Windows.
  • Removed the npx path.
  • Synced wording and code blocks between README and release body.

^{Written for commit 759706d. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files