Add copilot instructions (#3470)

Author: nkolev92

.github/copilot-instructions.md

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+# .NET Documentation Guidelines
+
+## Disclosure
+
+For any Markdown files generated by AI, always disclose that they were created with the assistance of AI. Add the following frontmatter key/value pair:
+
+```markdown
+ai-usage: ai-generated
+```
+
+## Terminology
+
+Unless otherwise specified, all .NET content refers to modern .NET (not .NET Framework).
+
+## Writing Style
+
+Follow [Microsoft Writing Style Guide](https://learn.microsoft.com/en-us/style-guide/welcome/) with these specifics:
+
+### Voice and Tone
+
+- Active voice, second person addressing reader directly.
+- Conversational tone with contractions.
+- Present tense for instructions/descriptions.
+- Imperative mood for instructions ("Call the method" not "You should call the method").
+- Use "might" instead of "may" for possibility.
+- Use "can" instead of "may" for permissible actions.
+- Avoid "we"/"our" referring to documentation authors or product teams.
+
+### Structure and Format
+
+- Sentence case headings (no gerunds in titles).
+- Be concise, break up long sentences.
+- Oxford comma in lists.
+- Use bullets for unordered lists.
+- Number all ordered list items as "1." (not sequential numbering like "1.", "2.", "3.", etc.)
+- Ordered and unordered lists should use complete sentences with proper punctuation, ending with a period if it's more than three words.
+- Avoid "etc." or "and so on" - provide complete lists or use "for example".
+- Use "for example" instead of "e.g.".
+- Use "that is" instead of "i.e.".
+- No consecutive headings without content between them.
+
+### Formatting Conventions
+
+- **Bold** for UI elements.
+- `Code style` for file names, folders, custom types, non-localizable text.
+- Raw URLs in angle brackets.
+- Use relative links for files in this repo.
+- Remove `https://learn.microsoft.com/en-us` from learn.microsoft.com links.
+
+## File Naming
+
+New Markdown files: lowercase with hyphens, omit filler words (the, a, etc.).
+
+## Special Cases
+
+- When you (Copilot) are assigned an issue in GitHub, after you've completed your work and the workflows (status checks) have run, check to make sure there are no build warnings under the OpenPublishing.Build status check. If there are, open the build report (under View Details) and resolve any build warnings you introduced.