docs: simplify installation instructions to use go install

[Chriscbr]

Summary

• Replace multi-step clone-and-build installation instructions with a single go install github.com/DataDog/pup@latest command

Test plan

• Verify go install github.com/DataDog/pup@latest works correctly

🤖 Generated with Claude Code

[github-actions]

📊 Test Coverage Report

Overall Coverage: 75.1%

Threshold: 80% ❌

Coverage by Package

## Coverage by Package

- github.com/DataDog/pup/pkg/auth/callback/server.go:40: 81.2%
- github.com/DataDog/pup/pkg/auth/dcr/client.go:28: 100.0%
- github.com/DataDog/pup/pkg/auth/dcr/types.go:24: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/client.go:22: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/pkce.go:24: 85.7%
- github.com/DataDog/pup/pkg/auth/storage/factory.go:53: 94.7%
- github.com/DataDog/pup/pkg/auth/storage/keychain.go:44: 42.9%
- github.com/DataDog/pup/pkg/auth/storage/storage.go:58: 71.4%
- github.com/DataDog/pup/pkg/auth/types/types.go:23: 100.0%
- github.com/DataDog/pup/pkg/client/client.go:28: 94.1%
- github.com/DataDog/pup/pkg/config/config.go:22: 100.0%
- github.com/DataDog/pup/pkg/formatter/formatter.go:31: 100.0%
- github.com/DataDog/pup/pkg/util/time.go:20: 95.8%

## Summary

total:								(statements)		75.1%

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📈 Coverage Status: ❌ FAILED - Coverage below minimum threshold

_{Updated for commit f381012}