Add certificate rotation instructions and examples

Currently page "Certificate Rotation" [1] doesn't contain any information about how to perform the rotation. We need to provide specific steps on how to rotate root certificate, intermediate certificate and node certificate.

Alternatively, we can split "Configuring Server Certificates" [2] into two pages: "Certificate Generation" and "Uploading and Rotation of Certificates" (since not all the customers need help with how to generate the certs, but uploading is very "couchbase" specific procedure; at the same time uploading and rotating are very similar so we can have one page for them). 

[1] https://docs.couchbase.com/server/6.5/manage/manage-security/rotate-server-certificates.html

[2] https://docs.couchbase.com/server/6.5/manage/manage-security/configure-server-certificates.html