[DRAFT] Implement alternate SM install journey

[conceptualshark]

Description

⚠️ draft - do not merge

This PR will be for soliciting feedback on alternate setups + next steps for the SM installation user journey,

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, @conceptualshark! 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/self-managed/about-self-managed.md
• docs/self-managed/setup/install.md
• docs/self-managed/setup/overview.md
• docs/self-managed/setup/upgrade.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.

[conceptualshark]

@theburi I don't think this represents the final state, just an initial cutover to a new structure before we introduce any content changes. Here's a few outstanding items:

• Right now, "Reference architecture" is nested under "Deploy," but I think these could be better merged as the content is updated.
• In the initial miro draft, "Deploy" was not recommended, but I still think it makes more sense for the user experience. Users in the SM docs are going to be looking for instructions on how to deploy a SM distribution; they may not recognize that there is any overlap with Deploying in Modeler, as their use cases are different. I think the audiences will be sufficiently different to know what they're looking for here, and Deploy is the most descriptive. We also currently use Deploy as a section under Setup, and haven't had any reported confusion there yet.
• I'd like to create a few more landing pages, likely in a follow-up PR: some summary content for getting started, update the installation landing page, a new/updated Deploy landing page, and one for Update to describe the difference between the two guides here (Helm vs not Helm).
• Nothing in operational guides, components, etc, is touched yet in this PR - only the initial install/onboarding experience.

[github-actions]

The preview environment relating to the commit 7a6fb94 has successfully been deployed. You can access it at https://preview.docs.camunda.cloud/pr-4601/index.html