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

[nicolasiscoding]

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.