doc: update RGW HTTPS configuration to use certmgr and new fields

doc: update RGW HTTPS configuration to use certmgr and new fields

With the introduction of certmgr, RGW services now support three
certificate sources: cephadm-signed (default), inline, and reference.
Docs have been updated to:

- Show how to provide inline certificates using the new ssl_cert/ssl_key
  fields instead of the deprecated rgw_frontend_ssl_certificate.
- Explain how to register and reference user-provided certs/keys
- Clarify that cephadm-signed certificates remain the default, with
  optional wildcard SANs support.

The usage of rgw_frontend_ssl_certificate is still supported for
backward compatibility, but is now documented as deprecated.

Signed-off-by: Redouane Kachach <rkachach@ibm.com>