[Docs] Certificate Authority Setup

[bagasme]

The guide is based on OpenSSL CA Guide.

This can be applied in intranet situation, when using internal CA seems more appropriate than using external one.

[codecov-io]

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[techknowlogick]

I think this is out of scope of what should be in the docs as this isn't specific to Gitea,

[bagasme]

@techknowlogick let's close now.