NGINX One Console: based on feedback, clarify process for certs

[mjang]

Is your feature request related to a problem? Please describe

The current docs need to cover:

• Two different kinds of certificates
  • For NGINX deployments
  • Associated with license keys (subscriptions)

For the following use cases:

• Instances that are not already part of a Config Sync Group
• Config Sync Groups that may (or may not) include instances

Describe the solution you'd like

Address how our users would look in our docs, based on ToC

• When users look for a discussion on certificates, they'd look to Manage certificates. To that page, we need to add:
  • Different types of certs in question
    • Certs for deployments (typically with /etc/ssl/nginx/nginx-repo.crt and nginx-repo.key)
    • Certs associated with the license key (example that will be shown in the UI: /etc/nginx/server.crt and server.key)
  • Different use cases for deployments
    • Instance
    • Config Sync Group
• We currently doc what to do to add / edit certs for an instance, in https://docs.nginx.com/nginx-one/how-to/nginx-configs/add-file/
  • The file is currently under "Instances and Configurations"
  • We need (mostly) the same content under "Config Sync Groups" (next to "Manage Config Sync Groups")
  • Process: set up "includes" per our style guide, https://github.com/nginx/documentation/blob/main/templates/style-guide.md#guidelines-for-includes
  • (Maybe: describe the source of the include? Probable location: content/nginx-one/)
  • Note: the UI limits the cert keys that you can include, especially the location
    • Describe how NGiNX One Console detects existing keys in /etc/ssl/nginx
    • Note how you can write keys only to config-dirs --> which may be changed
    • Related: Update docs per PR 882 agent#965, PR 882