doc: add openbao.md

[eyenx]

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Issue reference

[marcduiker]

Hi @eyenx, thanks for contributing this. Can you point to a minimum implementation that uses this? I'd like to see it working before I approve & merge.

[eyenx]

Hi @marcduiker let me try to address your comments this week

[msfussell]

@eyenx - Ping on this. Thanks

[eyenx]

I know I'm sry. Trying to get it done before the holidays. Let's see.