Add a docs README and link checker

Add tools/check_docs.py to require READMEs only in tracked docs directories,
validate local links and verify every tracked image is linked. Gitignored
generated files such as realism runs are exempt. Document the command and add
unit tests. Python 97/97.

docs/getting-started.md

@@ -20,6 +20,9 @@ godot --headless --path . --quit-after 120
 # Run all unit tests headlessly (exit code 0 = pass)
 godot --headless --path . --script res://src/test/test_runner.gd
 
+# Check docs README coverage, tracked images and local links
+python3 tools/check_docs.py
+
 # Save a screenshot to user:// (windowed)
 godot --path . --script res://src/test/take_screenshot.gd