Add Copilot instructions (#8189)

Author: vladaskorohodova

.github/copilot-instructions.md

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+# GitHub Copilot Instructions
+
+These instructions summarize the DevExpress/devextreme-documentation repository so you can make efficient, correct suggestions with minimal exploratory searching. Trust these instructions first; search only if certain information is missing or proves inaccurate.
+
+This repository contains source Markdown files, embedded sample applications, images, and reusable include fragments for the public DevExtreme documentation site: https://js.devexpress.com/Documentation/.
+
+This repository is a content + light tooling project (not a compiled application). Your typical changes will involve:
+- Reviewing style consistency and grammar of documentation content. The documentation content is straightforward technical writing targeting a software developer audience.
+- Reviewing link/reference consistency and accuracy in documentation content.
+- Reviewing style consistency of code samples (JavaScript/HTML/CSS/TypeScript).
+- Reviewing technical accuracy of code samples (JavaScript/HTML/CSS/TypeScript).
+
+What you need to do:
+- You will review Pull Requests (PRs) submitted by contributors.
+- You will leave comments on PRs when you spot issues.
+What you will not do:
+- YOU WILL NOT SUBMIT PRS YOURSELF.
+- YOU WILL NOT WRITE NEW DOCUMENTATION CONTENT YOURSELF.
+
+When reviewing PRs, please follow these guidelines:
+
+When reviewing documentation content:
+1. Follow American English spelling and grammar.
+2. Use active voice and present tense. Prefer active sentences with clear subject, object, and verb.
+3. Use consistent terminology (refer to existing documentation).
+4. Do not use contractions and elisions; prefer full/complete wording versions.
+5. All the headings should use Title Case. Do not use end punctuation (periods) at the end of headings.
+6. Remember to write alt text for images.
+7. Use the Oxford comma.
+8. Avoid introductory dependent clauses followed by commas (e.g., prefer "The function returns true" over "When called, the function returns true").
+
+When reviewing code samples:
+1. Follow the existing code style in the file.
+2. Ensure code samples follow best practices for the relevant technology. For example, Vue samples should use the Composition API + TS. React samples should use functional components + hooks + TS.