Update Documentation to include lease_threshold_renewal clarity

[jj-hcp]

Also updated to include PKICert documentation that shows using "writetoFile" such that practitioners can reference it while using it for the Vault Agent

Description

What does this PR do?

TODO only if you're a HashiCorp employee

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[VioletHynes]

and the certificate will rotate at 70%

Is the 70% related to this configuration at all? For example, by default, this is set to 0.9, would it be 90% by default?

[jj-hcp]

updated

[VioletHynes]

Looks good to me! Thanks for this :)