fix(doc): fix secret manager section header levels #4060

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@breun

The following contributors were found for this pull request:

✅ 95422b0 PR Opener: @breun
✅ 4a9efd3 Author: <nb*****se@bol.com>
✅ 95422b0 Author: @zhumin8 <zh***n@google.com>

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