Expand on custom public certificate trust usage

What does this MR do?

This MR seeks to add further clarification when installing custom public SSL certificates.

1. There is currently an introductory statement of:

   Some environments connect to external resources for various tasks. Omnibus GitLab allows these connections to use HTTPS, and supports connections with self-signed certificates.

   However, we then provide instructions surrounding the context of installing custom public SSL certificates for GitLab itself as a listener, and not necessarily "connecting to external resources". This MR seeks to further expand on these different cases.

2. An example has been added to the WARNING: alert box regarding splitting custom certificate chains into separate files.

Related issues

• This originally came about via discussions with a customer where there was some confusion surrounding this part of our documentation, particularly where the specific example steps are limited to the context of "trusting custom public SSL certificates where the certificates are applied to GitLab itself" vs "trusting custom public certificates for external resources GitLab needs to connect to".

  • Zendesk ticket (internal use only)

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