Add `asserts` example in Handbook

Suggestion

🔍 Search Terms

List of keywords you searched for before creating this issue. Write them down here so that others can find this suggestion more easily and help provide feedback.

asserts, narrowing, property

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Add asserts examples in the handbook to be properly indexed in the Search box. If there is not any specific reason for the absence of this feature in the examples I could make a PR

📃 Motivating Example

Currently asserts exists only in release notes of 3.7 and cannot find examples in handbook.

💻 Use Cases

Have asserts indexed in search box and exist in handbook, so we know that this functionality exists, without finding randomly through google searches.