Update preview mode docs #8877

[alhambrav]

Ticket reference or full description of what's in the PR

Update preview mode docs craftersoftware/craftercms#8877

Summary by CodeRabbit

• Documentation
  • Improved readability of Freemarker and Groovy variable reference tables.
  • Clarified how to detect Preview Mode using the modePreview variable.
  • Added Groovy and FreeMarker usage examples for Preview Mode checks.
  • Expanded Preview Mode security guidance with practical scripting examples.

[coderabbitai]

Walkthrough

The documentation converts variable tables to reStructuredText list tables, updates modePreview descriptions, and adds Groovy and FreeMarker examples for preview-only logic.

Changes

Preview mode documentation

Layer / File(s)	Summary
Variable reference updates source/includes/freemarker-variables.rst, source/includes/global-groovy-variables.rst	Variable tables now use list-table syntax. The modePreview descriptions explain preview-mode checks.
Preview mode examples source/reference/api/groovy-java-api.rst, source/reference/modules/engine.rst	The documentation shows Groovy and FreeMarker conditions that use modePreview.
Estimated code review effort: 1 (Trivial)	~5 minutes

Possibly related issues

• Update docs preview mode sections craftercms#8877 — Requests preview-mode documentation updates for Groovy and FreeMarker, including modePreview examples.

Possibly related PRs

• craftersoftware/docs#2102 — Modifies the same Preview Mode section in source/reference/modules/engine.rst.

Suggested reviewers: sumerjabri

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description includes the required heading and provides a clear summary with the issue reference.
Title check	✅ Passed	The title clearly identifies the primary change: updating preview mode documentation.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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