Create copilot-instructions.md file (#55839)

Author: hubwriter

.github/copilot-instructions.md

@@ -0,0 +1,17 @@
+This documentation repository consists mainly of content written in Markdown format. These files are converted into HTML for displaying on a website. Most Markdown files become a single article on the documentation site. Other files contain reusable content which is inserted into multiple articles. The repository also contains YAML files (e.g. for variable text), image files, JavaScript/TypeScript files, etc.
+
+### Using variables
+
+Within Markdown files, with the exception of the `title` field in the metadata at the start of a file, always use the Liquid syntax variables rather than text if a variable has been defined for that text. For example:
+
+| Use this variable | Don't use this text | File where variable is defined |
+| --- | --- | --- |
+| `{% data variables.product.prodname_dotcom %}` | GitHub | data/variables/product.yml |
+| `{% data variables.product.prodname_ghe_server %}` | GitHub Enterprise Server | data/variables/product.yml | 
+| `{% data variables.product.prodname_copilot_short %}` | Copilot | data/variables/product.yml | 
+| `{% data variables.product.prodname_copilot %}` | GitHub Copilot | data/variables/product.yml | 
+| `{% data variables.copilot.copilot_code-review_short %}` | Copilot code review | data/variables/copilot.yml | 
+| `{% data variables.enterprise.prodname_managed_user %}` | managed user account | data/variables/enterprise.yml | 
+| `{% data variables.code-scanning.codeql_workflow %}` | CodeQL analysis workflow | data/variables/code-scanning.yml | 
+
+There are many more variables. These are stored in various YAML files within the `data/variables` directory.