Move direct deployment engine to public preview

[denik]

Changes

• Move the direct deployment engine status from "experimental" to "public preview".
• Remove the experimental warning from databricks bundle deployment migrate command.
• Update docs/direct.md status line.

Why

The direct deployment engine has matured enough for public preview. Removing the experimental warnings reflects this and encourages broader adoption beyond development-only targets.

[github-actions]

Suggested reviewers

Based on git history of the changed files, these people are best suited to review:

• @andrewnester -- recent work in ./
• @pietern -- recent work in ./, cmd/bundle/deployment/, docs/

Confidence: high

Eligible reviewers

Based on CODEOWNERS, these people or teams could also review:

@anton-107, @shreyas-goenka, @simonfaltum

_{Suggestions based on git history of 3 changed files (3 scored). See CODEOWNERS for path-specific ownership rules.}