Add a stub documentation that gives a brief overview of AST analysis

Author: Razoric480

docs/analysing_ast.md

@@ -0,0 +1,68 @@
+# Analyzing student code with the GDScript AST
+
+The app uses a custom version of the Godot engine that exposes the GDScriptParser's parse tree.
+
+## Access
+
+1. Create a `GDScriptErrorChecker`
+2. Call `set_source()` and provide the source code. The code should be a valid, parsable class.
+3. Provided there are no parser, analyzer, or compiler errors, you can then call `get_root_parse_node()`, which returns a `GDClassNode`.
+
+`set_source()` will return `OK` if there are no parser, analyzer, or compiler errors.
+
+If you mean to analyze the parser nodes outside of the scope where you set the checker up, make sure to keep a reference to it. Once the `GDScriptErrorChecker` has its RefCount go to 0, it will clear the parser and all the nodes, and you will get garbage data out of the GDNodes at best.
+
+### Low level access
+
+The most raw way of analyzing student code is to directly access the functions in the root `GDClassNode`, like `get_member(function_name)` to get a `GDFunctionNode`.
+
+See the in-editor documentation for the individual `GDNode` RefCounted objects for the API.
+
+### High level access
+
+The `GDScriptASTAnalyzer` class is available to make accessing data easier without worrying about the API as much.
+
+```gdscript
+var checker = GDScriptErrorChecker.new()
+checker.set_source(class_code)
+var root := _checker.get_root_parse_node()
+var analyzer = GDScriptASTAnalyzer.new(root)
+```
+
+From there, you can access friendly functions like `analyzer.get_function_named(function_name)` and `analyzer.get_function_parameter_name(function, parameter_index)`.
+
+#### Expression system
+
+If you want to check that the tree has a particular shape and uses particular patterns, you can use the `GDExpr` class' static functions to build a comparator tree.
+
+For example, the expected code is
+
+```gdscript
+func run():
+	var health = 100
+
+	if health > 5:
+		print("health is greater than five.")
+```
+
+To verify it, I can thus write an expression against it:
+
+```gdscript
+func test_statement_is_true() -> String:
+	var run_function := _analyzer.get_function_named("run")
+	
+	# suites are the bodies of functions, loops, match branches, etc
+	if not GDExpr.suite(
+        # if
+		GDExpr.if_block(
+            # health > 5
+			GDExpr.bin_op(
+				GDExpr.identifier("health"),
+				GDExpr.literal(5),
+				GDBinaryOpNode.OP_COMP_GREATER
+			)
+		)
+	).matches(run_function):
+		return tr("The comparison is not correct. Did you use the right comparison?")
+	return ""
+```