Bug 1789670 - Add some basic layout debugger documentation. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D156703

layout/docs/LayoutDebugger.md

+# Layout Debugger
+
+## Overview
+
+The layout debugger can dump a variety of things including:
+- Matched CSS Rules
+- Text Runs
+- Frames
+
+## Usage
+- Use a debug build or add `ac_add_options --enable-layout-debugger` to your mozconfig.
+- Run with `./mach run --layoutdebug [filename]`
+

layout/docs/index.rst

@@ -12,3 +12,4 @@ contains general information about layout and the layout team at Mozilla.
 
    AccessibleCaret
    Reftest
+   LayoutDebugger