docs: write user-facing documentation for bulk mail merge feature

Description

Write user-facing documentation for the new Bulk Mail Merge feature (mail merge with TurboSign signature support).

Scope

Cover the full end-to-end workflow:

1. Overview — what bulk mail merge is, when to use it (e.g., employee hire letters, client agreements)
2. Prerequisites — template with variables, CSV/Excel spreadsheet with data columns, TurboSign enabled (for signature fields)
3. Step-by-step wizard walkthrough:
   • Upload: drag-and-drop spreadsheet upload, supported formats (CSV, XLSX)
   • Column Mapping: auto-matching, manual mapping via dropdowns, unmapped variable warnings
   • Signature Configuration: toggling variables as signature anchor fields, recipient slot configuration (email + name columns), field types (Signature, Initial, Date, Name, etc.)
   • Review & Submit: preview of generated documents, batch submission
4. Document naming patterns — how the document name builder works (column values + custom text segments)
5. Job status tracking — how to monitor batch progress
6. Limitations — max recipients per document, max rows per batch, supported file types

Related PRs

• Backend: nicolasiscoding/RapidDocxBackend#1260
• Frontend: nicolasiscoding/RapidDocxFrontEnd#2023
• Worker: TurboDocx/TurboDocxSignatureServiceWorker#7
• Feature issue: nicolasiscoding/RapidDocxBackend#1267

Notes

Screenshots should be captured from the staging environment once all three PRs are merged and deployed.

Review the backend PR #1260, frontend PR #2023, worker PR #7, and feature issue #1267 to verify the workflow, supported formats, limits, and job states. Capture staging screenshots after all three PRs are merged and deployed, then document the complete wizard flow, naming patterns, status tracking, prerequisites, and limitations.