Fix broken link to contributing guidelines (#3587)

The original URL (https://navigation.ros.org/contribute/index.html) seems not to exist, returning an HTTP 404. Hence, I've replaced the link with a page that seems most relevant.

README.md

@@ -16,7 +16,7 @@ For detailed instructions on how to:
 - [Navigation Plugins](https://navigation.ros.org/plugins/index.html)
 - [Migration Guides](https://navigation.ros.org/migration/index.html)
 - [Container Images for Building Nav2](https://github.com/orgs/ros-planning/packages/container/package/navigation2)
-- [Contribute](https://navigation.ros.org/contribute/index.html)
+- [Contribute](https://navigation.ros.org/development_guides/involvement_docs/index.html)
 
 Please visit our [documentation site](https://navigation.ros.org/). [Please visit our community Slack here](https://join.slack.com/t/navigation2/shared_invite/zt-hu52lnnq-cKYjuhTY~sEMbZXL8p9tOw) (if this link does not work, please contact maintainers to reactivate).