doc: add summary for core concepts

website/advanced/core-concepts.md

@@ -152,3 +152,8 @@ We have to preserve some trivial nodes for precise matching.
 Tree-sitter parsers do not encode all semantics with named nodes. For example, `class A { get method() {} }` and `class A { method() {} }` are equivalent in Tree-sitter's AST. The critical token `get` is not named nor has a field name. It is a trivial node!
 
 If you do not care about if the method is a getter method, a static method or an instance method, you can use `class $A { method() {} }` to [match all the three methods at once](https://ast-grep.github.io/playground.html#eyJtb2RlIjoiUGF0Y2giLCJsYW5nIjoiamF2YXNjcmlwdCIsInF1ZXJ5IjoiY2xhc3MgJEEgeyBtZXRob2QoKSB7fSB9IiwiY29uZmlnIjoicnVsZTpcbiAga2luZDogcGFpclxuICBoYXM6XG4gICAgZmllbGQ6IGtleVxuICAgIGtpbmQ6IHN0cmluZyIsInNvdXJjZSI6ImNsYXNzIEEgeyBtZXRob2QoKSB7fX1cbmNsYXNzIEIgeyBnZXQgbWV0aG9kKCkge319XG5jbGFzcyBDIHsgc3RhdGljIG1ldGhvZCgpIHt9fSJ9). Alternatively, you can [fully spell out the method modifier](https://ast-grep.github.io/playground.html#eyJtb2RlIjoiUGF0Y2giLCJsYW5nIjoiamF2YXNjcmlwdCIsInF1ZXJ5IjoiY2xhc3MgJEEgeyBnZXQgbWV0aG9kKCkge30gfSIsImNvbmZpZyI6InJ1bGU6XG4gIGtpbmQ6IHBhaXJcbiAgaGFzOlxuICAgIGZpZWxkOiBrZXlcbiAgICBraW5kOiBzdHJpbmciLCJzb3VyY2UiOiJjbGFzcyBBIHsgbWV0aG9kKCkge319XG5jbGFzcyBCIHsgZ2V0IG1ldGhvZCgpIHt9fVxuY2xhc3MgQyB7IHN0YXRpYyBtZXRob2QoKSB7fX0ifQ==) if you need to tell getter method from normal method.
+
+## Summary
+Thank you for reading until here! There are many concepts in this article. Let's summarize them in one paragraph.
+
+ast-grep uses tree-sitter to parse _textual_ source code into a detailed tree _structure_ called **CST**. We can get **AST** from CST by only keeping **named nodes**, which have kinds. To search nodes in a syntax tree, you can use both node **kind** and node **field**, which is a special role of a child node relative to its parent node. A node with either a kind or a field is a **significant** node.