Improve copilot instructions

Author: samdark

.github/copilot-instructions.md

@@ -1,2 +1,24 @@
+# Yii3 Definitive guide, cookbook, and other docs
+
+The project is the source of truth for Yii3 documentation and tips. It is maintained by the Yii team and community.
+
+The documentation is written in GitHub flavor Markdown format.
+
+```
+cookbook/en/ - cookbook containing individual recipes about doing things with Yii3.
+guide/en/ - guide containing step-by-step instructions for learning Yii3 and covering some conceptual topics.
+*.md - some internal docs and top level policies.
+```
+
+Yii3 consists of [multiple packages and respective repositories](https://github.com/topics/yii3). 
+Each package has its own documentation that could be found in its repository and should be used to learn
+about the package.
+
+The documentation is following a Microsoft Docs style guide which is checked with Vale Linter GitHub Action. Results
+are available after the action is completed.
+
+Rules are:
+
 - Edits are permitted only in `cookbook/en`, `guide/en` and `.md` files in root directory.
 - Conform to `014-docs.md` except translations part.
+- Follow individual package documentation when possible.