[TASK] List all secrets and clarify how to set secrets in our docs

[hisImminence]

Related to

We got feedback from a customer that indicates our documentation about how to configure secrets is not good enough. We should make the following clear:

• List all possible secrets
• How to generate k8s secrets
• How to reference each secret in our Helm Charts (as naming is not consistent)

Context Suggestion

1. General part about secret management
2. We can classify 2 types of secret in our helm charts:
   * Autogenerated secrets
      * If autogenerate secrets is set to true, all secrets used between C8 applications are autogenerated (e.g. keycloak needs to have a clientId/ secret for each application operate/ optimize, ...)
      * This is not set for true by default
      * We should list all of this kind of secrets.
      * Users also can create their own secrets and reference them if preferred (disable autogeneration), or replace them.
      * Even if passwords are auto generated, the name and keys still need to be referenced in the values.yaml

   * Secrets for external services
      * These secrets are used to configure external services outside of C8 applications, e.g. email smtp for webmodeler, external ES/ OS...
      * These can not be autogenerated as they depend on secrets created by customers.
      * customer needs to set them in their values files 
      * Document a list of secrets for external services