docs(user-guide): document workflow template placeholder variables

[bohdanmaliar]

Summary

Documents the new placeholder-variable feature for workflow templates (EPMCDME-10585): authors can mark values in a template's YAML with ${input:variable_name}, and users are prompted once per unique placeholder via a Configure Template Variables dialog when creating a workflow from that template.

Changes

• Extended create-workflow-from-template.md with a new step describing the Configure Template Variables dialog (including a screenshot) and a "Using Placeholder Variables in Templates" section explaining the ${input:variable_name} syntax with a real example
• Added a cross-reference note in configuration/introduction.md distinguishing the creation-time ${input:...} syntax from the existing execution-time {{variable_name}} syntax
• Added two FAQ entries: how to use placeholder variables, and the difference between ${input:...} and {{variable_name}}

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets (<text> must be `<text>`)
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation

Additional Notes

Screenshot added: docs/user-guide/workflows/images/configure-template-variables-dialog.png