Clarify third-party link guidelines

src/frontend/src/content/docs/community/contributor-guide.mdx

@@ -235,6 +235,16 @@ When contributing to `aspire.dev`, follow these writing guidelines to ensure con
 - **Follow formatting conventions** - Use consistent formatting for code snippets, commands, and technical terms. Refer to the examples in this guide for guidance.
 - **Review existing content** - Before adding new content, review existing documentation to avoid duplication and ensure coherence.
 
+### Third-party links
+
+Third-party links can provide useful context, but they must serve the reader rather than the linked organization:
+
+- Include third-party links only when they are directly relevant, and use them sparingly. Prefer Aspire documentation when it covers the same information.
+- Describe third-party resources factually and neutrally. Avoid endorsements, marketing language, promotional comparisons, and calls to purchase or sign up.
+- Link to technical documentation or other substantive resources, not exclusively to a paid product, pricing, lead-generation, or sales page.
+- Do not add content whose purpose is to promote a product or service that competes with Aspire.
+- Disclose any material affiliation with the linked organization, such as employment, sponsorship, or ownership, in the pull request. An affiliation does not automatically disqualify a link, but the link and surrounding content must meet the same relevance and editorial standards as any other contribution.
+
 ### AppHost-specific C# and TypeScript content
 
 When you are documenting AppHost-specific content that changes between C# and TypeScript, use synced `Tabs` and `TabItem` blocks with `syncKey='aspire-lang'`. Each code snippet should have its own C# and TypeScript selector, and the shared sync key keeps the selected language consistent across the page.