docs: remove alpha feature nature from C8 Query API

[tmetzke]

Description

Remove the alpha documentation from the C8 REST Query API.

closes #4761

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, @tmetzke! 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/apis-tools/camunda-api-rest/camunda-api-rest-overview.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.

[pepopowitz]

1. In terms of sequencing, we should probably wait to merge this PR until Remove alpha label from query API in OpenAPI spec camunda#26022 is complete and the API spec has been updated here in the docs.
2. @tmetzke I interpret this PR to mean that we do not intend to have alpha features in the API in the future. Is that accurate? If it's not, and it's more a case of we've graduated all current alpha endpoints, then I think we should leave this explanation in the docs for when another alpha endpoint comes.
3. If we are indeed done with alpha endpoints, then there is also some logic I can remove in the API generation process (which docusaurus-ifies the alpha notes in the spec). I will take care of this.