Docs: update Configure GitLab using an external PostgreSQL service

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Context

Follow up from https://gitlab.com/gitlab-org/gitlab-ee/issues/4222

Slack thread: https://gitlab.slack.com/archives/C0AR2KW4B/p1513012633000010

What needs update

This doc: https://docs.gitlab.com/ee/administration/external_database.html has some issues:

• Using an external PostgreSQL service is CE/EE, but there's no CE doc
  • Add CE doc
• PostgreSQL HA is EEP
  • Update step 3 to reflect that
• Add note at beginning informing which version of GitLab introduced this feature, linking to the original MR
• Add note explaining which part is available in CE/EES/EEP
• Link it from https://docs.gitlab.com/ee/administration/#installing-gitlab
• Minor edit: jump line after the title (h1)