docs: improve template variables documentation with variable reference page

[anttiviljami]

Summary

• Add new Variable Reference page with comprehensive Handlebars helper documentation
• Streamline main template-variables page as an overview with links to the reference
• Add Environments & Secrets UI screenshot to the environments-secrets page
• Document {{ env.* }} prefix for environment variables in templates prominently

Changes

New: Variable Reference Page (docs/templates/variable-reference.md)

Complete reference for all template variables and Handlebars helpers including:

• Entity variables syntax
• System variables ({{system.date}}, {{system.time}})
• Environment variables with {{ env.* }} prefix
• Organization and user variables
• Portal & link variables
• All Handlebars helpers (address, contact, date/time, currency, comparison, boolean, tag filtering, string)
• Excel-like calc formulas with all arithmetic operators, math functions, and date functions
• "Where Variables Are Supported" feature matrix

Updated: Template Variables Page (docs/templates/template-variables.md)

• Streamlined as an overview page
• Links to the new Variable Reference for detailed helper documentation
• Quick start examples for common use cases

Updated: Environments & Secrets Page (docs/environments/environments-secrets.md)

• Added UI screenshot showing the Environments & Secrets configuration page

Context

This improves the documentation based on user feedback about finding information on template variables, particularly:

• Where {{system.date}} is documented
• How to use environment variables in templates with {{ env.* }}
• Complete list of available Handlebars helpers

Slack: https://epilot.slack.com/archives/CG5GDRC77/p1777465683037349?thread_ts=1777464857.029149&cid=CG5GDRC77

Test plan

• Preview the docs site to verify all pages render correctly
• Check that internal links work between pages
• Verify the screenshot displays properly

https://claude.ai/code/session_01EMdAbE7cPq8KFERXxAbgp2