Add documentation for using internal CA

[mnaser]

There are use cases where users would want to use their own CA, such as in #30.

We are already able to handle those cases today, however, there is no documentation to explain this. We should update our documentation here:

https://github.com/vexxhost/atmosphere/blob/main/docs/certificates.md

In order to include another section on how to use a pre-existing CA which we can use to sign certificates.

[okozachenko1203]

i am on it