docs: new page with mdbook (#8342)

- Use mdbook to generate a book from collection of documents
 - documents are
     - copied and processed from READMEs
     - original content stored in the docs_src folder
     - markdeep docs transformed into embedded html
 - Main logic is in script docs_src/build/run.py

docs_src/src/notes/metal_debugging.md

@@ -0,0 +1,42 @@
+# Debugging Metal
+
+## Enable Metal Validation
+
+To enable the Metal validation layers when running a sample through the command-line, set the
+following environment variable:
+
+```
+export METAL_DEVICE_WRAPPER_TYPE=1
+```
+
+You should then see the following output when running a sample with the Metal backend:
+
+```
+2020-10-13 18:01:44.101 gltf_viewer[73303:4946828] Metal API Validation Enabled
+```
+
+## Metal Frame Capture from gltf_viewer
+
+To capture Metal frames from within gltf_viewer:
+
+### 1. Create an Info.plist file
+
+Create an `Info.plist` file in the same directory as `gltf_viewer` (`cmake/samples`). Set its
+contents to:
+
+```
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>MetalCaptureEnabled</key>
+    <true/>
+</dict>
+</plist>
+```
+
+### 2. Capture a frame
+
+Run gltf_viewer as normal, and hit the "Capture frame" button under the Debug menu. The captured
+frame will be saved to `filament.gputrace` in the current working directory. This file can then be
+opened with Xcode for inspection.