Add TLS docs

[andreasgerstmayr]

Added a document with an overview of our server TLS settings for each pod.

@rubenvp8510 is this doc accurate?

Also, a few questions:

• Should the gateway use the certificate and CA of spec.template.distributor.tls if configured?
  • if the gateway is enabled and spec.template.distributor.tls is configured, will it break gateway -> distributor connections?
• Currently there is no way to configure TLS for the gateway on non-OpenShift clusters?
• Should the TLS config be more flexible? allow choosing between
  • internal (self-signed by the operator)
  • serving-ca (generated by OpenShift)?
  • custom (ConfigMap and Secret)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 75.35%. Comparing base (83b6a17) to head (8f0d5a3).

Additional details and impacted files

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##             main     #813   +/-   ##
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  Coverage   75.35%   75.35%           
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  Files          89       89           
  Lines        6383     6383           
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  Hits         4810     4810           
  Misses       1343     1343           
  Partials      230      230           

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[rubenvp8510]

Added a document with an overview of our server TLS settings for each pod.

@rubenvp8510 is this doc accurate?

Also, a few questions:

* Should the gateway use the certificate and CA of `spec.template.distributor.tls` if configured?
  
  * if the gateway is enabled and `spec.template.distributor.tls` is configured, will it break gateway -> distributor connections?

I remember we prevent this when we generate the cofniguration, if the gateway is enabled TLS configuration is ignored., actually I think it will generate a bad configuration. https://github.com/grafana/tempo-operator/blob/main/internal/manifests/config/tempo-config.yaml#L12 I'll test today afternoon. I think if that is the case we need to add some validations to the webhook.

* Currently there is no way to configure TLS for the gateway on non-OpenShift clusters?

It seems is not :/

* Should the TLS config be more flexible? allow choosing between
  
  * `internal` (self-signed by the operator)
  * `serving-ca` ([generated by OpenShift](https://docs.openshift.com/container-platform/4.14/security/certificates/service-serving-certificate.html))?
  * `custom` (ConfigMap and Secret)

I think yes, in the case of self-signed vs custom, you can control this using the feature flags, wondering if we need to control this better on the CRD.