Add Retries section to README

[renaudhartert-db]

Summary

• Added a dedicated "Retries" section to the README, placed between "Paginated responses" and "Single-Sign-On (SSO) with OAuth"
• The README introduction already mentions that the SDK performs "intelligent retries," but there was no dedicated section explaining the retry behavior
• Added corresponding TOC entry and updated NEXT_CHANGELOG.md

Test plan

• Verify the new TOC entry links correctly to the Retries section
• Verify the Retries section renders correctly in GitHub markdown
• Verify NEXT_CHANGELOG.md has the documentation entry

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Inputs:

• PR number: 671
• Commit SHA: 1391e78bb69a9a05de9d2aad5e3619d88d4041f6

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