[DotNet] Add details to README in certificate samples

[JhontSouth]

#minor

Description

This PR adds more information to the README certificate samples to have an easier complete guide for SSL/TLS certificate configuration.

Proposed Changes

• Updated Readme of dotnet samples 84.bot-authentication-certificate and 85.bot-authentication-sni.

[tracyboehrer]

@JhontSouth For SN+I, I believe we can leave out the steps for creating a certifcate outside KeyVault. The reason being is that for MS folks, the cert needs to be from OneCert. In fact, creating a self-signed cert in KeyVault for SN+I just won't work. I will update the README once I have all the details. Basically they have to onboard with OneCert, and then they can create SN+I compatible certificates. Also, for SN+I, they can't upload the cert to the App Registration (I didn't notice if the READ still indicated that). It will cause a different alert if they do (pinning certs is not allowed either).

[JhontSouth]

Hi @tracyboehrer, we had this feedback pending. We changed the README's instructions to specify that the sample requires an existing certificate obtained through OneCert. We also removed the steps to upload the certificate to the app registration and replaced them with the manifest configuration, which is used for SN+I.
Let us know if there's anything else we need to change.
Thanks!