DO NOT MERGE - Split get started & guides

[akeller]

Description

Splits Spring guide into its own top level nav section.

☝️ Can consider loading this page without a sidebar since it's the only page in the section.

See https://camunda.slack.com/archives/C07CQC289JP for project team.

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.