Improve the documentation around Elasticsearch secure settings

[pebrc]

The current documentation is at https://www.elastic.co/guide/en/cloud-on-k8s/current/k8s-es-secure-settings.html

It can be very confusing to users, the key, path projection mechanism is not something users need to understand for basic usage but it is presented in one continuous paragraph with the basic use of the feature.

We could improve this as follows:

• have three sections maybe (titles tbd): basic usage, projection/subset usage, examples
• most users will learn all they need in the basic section and can ignore the complicated projection feature
• examples: we already link to a GCS example for snapshots, we could highlight this a bit more and add another example inline maybe for AWS S3 .