Add Insomnia to Postman alternatives

replace/postman.md

+---
+product: Postman
+type: API clients
+layout: replace
+---
+
+# Postman
+
+## Insomnia
+
+The Desktop API client for REST and GraphQL. Make requests, inspect responses.
+
+{% infobox %}
+ - **Website**: [insomnia.rest](https://insomnia.rest)
+ - **Source code**: [GitHub](https://github.com/Kong/insomnia)
+ - **License**: [MIT](https://github.com/Kong/insomnia/blob/develop/LICENSE)
+{% endinfobox %}
\ No newline at end of file