Document nebula CA & signing default validity lengths

[jasikpark]

Fixes #64

Another task might be to fully document all possible command line flags for nebula-cert and other commands

https://deploy-preview-65--nebula-docs-dn.netlify.app/docs/guides/quick-start/#creating-your-first-certificate-authority

This documents that CAs are valid for one year by default, that certificates are valid for one second before their CA expires, and both values can be adjusted by passing -duration XXhXXmXXs

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