Add RDBMS backup documentation

[stoerti]

Description

Preparation for RDBMS release. Will maybe not happen before Camunda 8.8

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.

[github-actions]

👋 🤖 🤔 Hello, @stoerti! Did you make your changes in all the right places?

These files were changed only in docs/. You might want to duplicate these changes in versioned_docs/version-8.6/.

• docs/guides/migrating-from-camunda-7/conceptual-differences.md
• docs/self-managed/concepts/databases/elasticsearch/elasticsearch-privileges.md
• docs/self-managed/concepts/databases/elasticsearch/opensearch-privileges.md
• docs/self-managed/concepts/databases/overview.md
• docs/self-managed/concepts/databases/relational-db/configuration.md
• docs/self-managed/operational-guides/backup-restore/backup-and-restore.md

You may have done this intentionally, but we wanted to point it out in case you didn't. You can read more about the versioning within our docs in our documentation guidelines.

[akeller]

Let me know if you need any help @stoerti 🙂

[akeller]

@stoerti thanks for your comment about delivery in the PR description. Since we are actively triaging changes for 8.7 and 8.8, I went ahead and added a target:8.8 label to this PR. We can always change this, but AFAIK, it will not ship with 8.7.