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/vulkan_debugging.md

@@ -0,0 +1,15 @@
+# Debugging Vulkan
+
+## Enable Validation Logs
+
+Simply install the LunarG SDK (it's fast and easy), then make sure you've got the following
+environment variables set up in your **bashrc** file. For example:
+
+```
+export VULKAN_SDK='/path_to_home/VulkanSDK/1.3.216.0/x86_64'
+export VK_LAYER_PATH="$VULKAN_SDK/etc/explicit_layer.d"
+export PATH="$VULKAN_SDK/bin:$PATH"
+```
+
+As long as you're running a debug build of Filament, you should now see extra debugging spew in your
+console if there are any errors or performance issues being caught by validation.