docs: fix a couple preview deploy incorrections

[pepopowitz]

Description

Updates the preview-env-deploy.yaml workflow to correct a couple small issues:

1. The build doesn't take nearly as long as it used to.
2. The preview link was taking you to a URL that contains two home pages stacked on top of each other 😱. I have no idea why this was happening. But everything is fine if we remove the index.html from the URL, which is how the real docs site would be navigated anyway. (We were originally including the index.html due to the preview environment infrastructure requiring it, but it doesn't seem to be required anymore.)

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support. (add bug or support label)
• This is already available but undocumented and should be released within a week. (add available & undocumented label)
• This is on a specific schedule and the assignee will coordinate a release with the Documentation team. (create draft PR and/or add hold label)
• This is part of a scheduled alpha or minor. (add alpha or minor label)
• There is no urgency with this change (add low prio label)

PR Checklist

• My changes are for an upcoming minor release and are in the /docs directory (version 8.8).
• My changes are for an already released minor and are in a /versioned_docs directory.

• I added a DRI, team, or delegate as a reviewer for technical accuracy and grammar/style:
  • Engineering team review
  • Technical writer review via @camunda/tech-writers unless working with an embedded writer.