Add documentation for all PDF tools

docs/tools/pdf-layers.md

@@ -0,0 +1,46 @@
+---
+title: PDF OCG Layers
+description: View, toggle, add, and delete Optional Content Group (OCG) layers in a PDF document.
+---
+
+# PDF OCG Layers
+
+Manage the Optional Content Groups (OCG) -- commonly called "layers" -- inside a PDF. You can toggle layer visibility, add new layers (including nested child layers), delete layers, and save the modified PDF.
+
+## How It Works
+
+1. Upload a PDF file.
+2. Click the process button to load the document with the PyMuPDF engine.
+3. The tool displays all existing layers with their visibility state, lock status, and nesting hierarchy.
+4. Toggle checkboxes to show or hide layers. Locked layers cannot be toggled.
+5. Add new top-level layers by typing a name and clicking the add button, or add child layers under an existing parent.
+6. Delete layers you no longer need.
+7. Click **Save** to download the modified PDF with your layer changes applied.
+
+## Features
+
+- Displays layer hierarchy with proper indentation for nested layers
+- Toggle layer visibility on or off
+- Locked layer indicator prevents accidental changes
+- Add top-level or child layers with custom names
+- Delete individual layers
+- Saves all changes into a new PDF file
+
+## Use Cases
+
+- Toggling visibility of markup layers in architectural or engineering drawings
+- Adding new annotation layers to a multi-layer design document
+- Removing obsolete layers from a template before distributing it
+- Inspecting which layers exist in a complex PDF received from a third party
+
+## Tips
+
+- If the PDF has no layers, the tool shows an empty state with a prompt to add one. Not all PDFs use OCG layers -- they are most common in CAD exports and design files.
+- Child layers are visually indented under their parent. You can nest layers to any depth.
+- Locked layers are displayed with a lock icon and their checkboxes are disabled. These locks are set by the PDF creator.
+
+## Related Tools
+
+- [View Metadata](./view-metadata)
+- [Edit Metadata](./edit-metadata)
+- [Organize & Duplicate](./organize-pdf)